One of the reasons TV Tropes is an effective gateway drug for new series and genres is its two-fold structure (the work pages and trope pages dichotomy), which creates a cumulative recommendation effect. The way it works, when a careful reader starts examining the tropes in their favorite works/genres of fiction (work plane), they soon discover a pool of their favorite tropes (trope plane) by determining the lowest common denominators of said works. Reading through the trope pages, in turn, inevitably makes them notice titles that, while unknown to them, keep popping up in examples of their favorite tropes. Eventually, they start reading said examples and, in doing so, create a sense of connection to the respective stories (especially if they also read spoilers). As the number of examples they read grows, the connection becomes more or less a commitment and reading/watching/playing the work of fiction in question becomes inevitable.
Another reason is that unlike reviews, advertising, and audience score aggregates, the trope descriptions and examples are personal and objective at the same time. Professional and amateur reviews are often personal but completely subjective, requiring both trusting the reviewer and liking their tastes for recommendation to take any effect. Advertising (especially trailers) is inherently geared towards promotion and while it can give powerful incentives, it just as often lies due to its very nature. Audience score aggregates may be objective (not guaranteed) but they are also completely impersonal, so they are rarely the decisive factor in choosing what to watch/play next. Wikipedia articles (and similar informative websites) often give you a clear idea what the story is about but they do not strive to make it palatable enough for you to want to enjoy it first-hand. TV Tropes, however, while not recommending or disapproving of anything per se, lets you discover reliable and objective reasons why you could like a given work of fiction—a method of decision making that is slower than the ones based on other sources but almost always guarantees zero disappointment in your choice.