In the late 90's, 3D polygons were ready to take over the gaming industry. A good chunk of the market at the time consisted of 2D Platform Games, so developers had the job to figure out a way for those obstacle courses to properly transition into a 3D space. The answer was to seemingly downplay the "obstacle course" part.
Enter the Collect-A-Thon (short for "Collection Marathon"). A style of Platform Game that was very popular in that era, inspired by the advent of Super Mario 64, which sought to take advantage of the new axis provided by 3D polygons through large, explorable worlds where players could move around freely. This, however, led to a new problem: traditional Level Goals at the end of linear paths looked a bit pointless in this new open-world reality. The solution was to turn exploration into a major part of the experience (if not, the main attraction), but that also meant having to incentivize the player with some sort reward that could also function as a goal in order to do so; that's were the collection aspect comes in.
The core formula is as follows: collect X number of Y things to advance further into the game. There are barriers, natural or not, that will prevent the player from just moving on to the next level unless they explore and collect enough of the resident Plot Coupons; however, very rarely are those conditions unfair (especially in the early game) and having to gather every collectible possible is never necessary in case you just want to finish the game, Golden Ending aside.
Collect-A-Thons are known to fill their worlds with trinkets of various types, each in their own tiers of importance. Primary collectibles are rare and the most important to get, always requiring one or more objectives of relatively moderate-to-hard difficulty to be completed before awarded (they're often shiny and/or gold, to highlight their importance). Secondary and tertiary collectibles, on the other hand, are plentiful and serve to directly or indirectly help the player obtain primary ones, either by outright granting said primary collectibles after enough of them are picked up, guiding the player towards points of interest or being used as "currency" for new moves and weapons that will be necessary in the long run. There are also "bonus" collectibles that are generally optional to a non-100% Completion run, meaning they can be either easier or harder to collect depending on how useful the reward provided is.
The genre would eventually lose popularity somewhere in the early 00's as games like Grand Theft Auto III would expand further on the open-world concept. That said, the notion of exploring worlds in order to find and collect things was not lost in many modern games. In fact, it still persists even two decades later. 3D Platformers, however, would fall to the wayside by the mid-00's just as 2D ones returned to viability in the market thanks to the success of New Super Mario Bros. and other games of the type that thrived on handhelds and even consoles. The term "Collect-A-Thon" itself is relatively recent (back in the 90's, those were just good old Platform Games, except in 3D) and sorta carries a derisive tone to it, but there are many who still utilize it unironically, especially for more modern takes of the genre.
Reminder that not every 3D Platformer is a Collect-A-Thon (following the modern, agreed-upon definition). They might borrow some elements that are part of the genre, like the Crash Bandicoot games that require collected crystals to progress, but unless they feature enough explorable areas and gated progression under a specific number of objects, they don't count. Similarly, 2D Collect-A-Thons do exist as well, in which case expect several branching paths and side-areas featuring important collectibles.
Compare Metroidvania, which also encourages exploration and employs gated progression, but is usually not divided in individual stages, instead featuring an interconnected world.
- Ape Escape
- Banjo-Kazooie series (Helped popularize the genre)
- Banjo-Kazooie: Grunty's Revenge
- Bubsy 3D
- A Bugs Life
- Conker's Bad Fur Day (Downplayed. There are some open areas at the beginning, but, from 1/3rd of the game onwards, it's more or less entirely linear and filled with scripted sequences)
- Donkey Kong 64 (Infamous for taking the genre Up to Eleven)
- Drawn to Life (The first game is a rare 2D example. The player has to retrieve book pages and missing creatures in order to complete levels, which are very explorable)
- Earthworm Jim 3D
- Fez (2.5D example with possibly the bare minimum to qualify. You just collect cubes [some of them divided into eight smaller cubes] and anti-cubes; your combined total of cubes and anti-cubes opens certain doors for progress, and determines which ending you get)
- Gex (Especially Enter The Gecko and Deep Cover Gecko)
- A Hat in Time
- Jak and Daxter: The Precursor Legacy (Sequels would abandon the genre altogether in favor of a Grand Theft Auto-like structure)
- The Legend of el Lobodestroyo Vs. la Liga de los Villanos
- Psychonauts (Zigzagged. The game has loads of collectibles, but progress is mostly independent of them. We say "mostly" because there are many instances with obstacles requiring abilities only obtainable under a specific rank. Collecting figments and challenge markers is necessary in order to rank up in the first place)
- Rocket: Robot on Wheels
- SpongeBob SquarePants: Revenge of the Flying Dutchman
- SpongeBob SquarePants: Battle for Bikini Bottom (Console version only. The GBA version was more of a standard 2D Platformer)
- The SpongeBob Movie Game (Console version only. While more linear than its predecessor, it still features optional areas to explore and gated progression in the form of new moves from Mindy, which are needed to finish their respective levels and require collecting a set number of Goofy Goober Tokens.)
- Spyro the Dragon
- Sly Cooper and the Thievius Raccoonus (Downplayed. Most levels are linear, but the hubs are explorable to an extent, similar to Spyro The Dragon. Sequels would feature open environments, but abandon the mandatory collecting)
- Super Lucky's Tale
- Super Mario 64 (Trope Maker and Codifier)
- Toy Story 2
- TY The Tasmanian Tiger (The first game is a straight forward collect-a-thon. The second and third games take on a more mission-based open world playstyle, but collecting is not mandatory. The fourth game ends up being a linear 2D platformer)
- Wario World (Zigzagged. Linear levels, but with plenty of explorable sidepaths containing mandatory and optional items)
- Yooka-Laylee (Spiritual Successor to Banjo-Kazooie. Created by former Rare devs)
- Yoshi's Story (Rare 2D Platform Game example. Levels endlessly loop and progress is achievable only by eating enough fruit found both in the main path and side areas)
- Yoshi's Crafted World (Progress gated by flowers collected. Levels feature branching paths)
- Zera: Myths Awaken