It's a TV pilot, maybe pilot movie, and a cast of characters is introduced. However, always one character whose job is to fill a specific role; dying before the pilot is over.
The meta reasons vary.
- To show how serious a situation it is.
- To make room for a designated main character.
- Because they're the first bad guy and have to die to show the scale of the threat.
- Because the writers want to prove that they're willing to kill people.
- Xander and Willow's friend Jesse McNally from Buffy the Vampire Slayer died to show how dangerous the vampires are and to make room for Buffy.
- Stargate Atlantis
- Col. Marshall Sumner, to show how dangerous the situation was are and to make room for Sheppard.
- The Wraith Keeper, as the first villain.
- Half of the crew of Voyager to show how dangerous the situation was are and to make room for the main cast.
- Cliffjumper is murdered by Starscream in the first episode of Transformers Prime to show that the Decepticons mean business.
- In the second episode of Green Lantern: The Animated Series, Shyir Rev sacrifices himself in order to buy his planet's colony enough time for them to escape from the Red Lanterns' world-destroying Liberator. His final words to Razer, who was previously shown to be having second thoughts about being a Red Lantern, help kick off the latter's Heel-Face Turn.