The scene tucked after a film's credits has moved from a rare joke to an expectation. The reason has more to do with how franchises are planned than with how films are written.
The scene is a scheduling tool
A studio building a connected series has to seed the next film long before that film is finished. A post-credits scene is the cheapest place to plant that seed without disturbing the story that just ended.
The main narrative can resolve cleanly, which matters because a film that ends on an unresolved hook tends to frustrate the people who paid for a complete story. The tease sits outside that contract.
It also costs very little. These scenes are often a single location, two actors and a day of shooting, sometimes captured during a gap in the main production schedule rather than as a separate shoot.
Credits length created the opportunity
Modern effects-heavy films carry enormous crew lists, and the credit roll on a large production runs for several minutes. That is dead time in a theatre where the audience is already leaving.
Filling it with something gives the roll a purpose and keeps a portion of the audience seated. Studios noticed that the practice was being rewarded with word of mouth about what people had stayed to see.
Why the tease outperforms a trailer
A trailer reaches a general audience that may have no attachment to the property. The people still seated after a film has ended are the most committed segment of that audience, already invested in the world.
Announcing a follow-up to that group generates conversation that spreads outward at no media cost. The information arrives as a discovery rather than as marketing, which changes how people pass it along.
That distinction is why studios keep the contents tightly guarded. A leaked post-credits scene loses the surprise that makes it travel, so prints and screeners are often withheld or altered.
The form has developed its own conventions
Two scenes became common, one placed immediately after the main credits and another at the very end. The first usually carries plot, while the second is more often comic and self-contained.
This split hedges against different audience behaviours. Viewers who stay only a moment still get something, and the ones who wait through the full roll are rewarded for the patience.
Where the convention starts to strain
The expectation now applies to films with no sequel planned, which produces scenes that gesture at a future nobody has committed to making. Those teases can age into loose ends.
Audiences have also learned to read them as obligations rather than surprises. When everyone waits, the discovery effect that made the device valuable in the first place quietly disappears.
Some filmmakers have responded by refusing the convention entirely, letting the credits roll on an ending that is meant to be final. That refusal now reads as a statement in itself.