A scene tucked after the credits costs very little to make and does work that the main body of a film cannot easily do. That combination explains why the device spread so widely.
It began as a reward for staying
The earliest examples were essentially jokes, aimed at the small group of viewers who sat through the crawl rather than leaving with everyone else.
Because the audience was self-selecting, the tone could be looser. Nothing after the credits has to serve the story, so it can be a gag, an outtake or a fragment.
That freedom is the origin of the form, and it survives in films that use the slot for humour rather than for plot.
Serialised storytelling needed a hand-off
Once studios began building connected series, they faced a problem: how to point at the next film without breaking the ending of this one.
A proper ending needs to close. Bolting a set-up onto the final scene undercuts the resolution the audience has just been given.
Placing the set-up after the credits keeps both. The film ends where it should, and the connection is made in a space that is formally outside it.
It is unusually cheap to produce
These scenes are typically short, use one or two performers and sit on an existing set or a plain background.
They are often shot separately from the main schedule, sometimes long after principal photography, which lets a studio add or replace one late.
That flexibility matters. A stinger can be swapped when a plan changes without touching the film that precedes it.
The conversation starts in the car park
A post-credits scene rarely explains itself. It shows an object, a face or a line whose meaning depends on knowledge the audience has to supply.
This produces discussion immediately after the screening, when people are together and still in the world of the film.
That discussion is marketing the studio does not pay for, and it is aimed squarely at the viewers most likely to buy a ticket for the next instalment.
Familiarity has blunted the effect
Audiences now expect the device, which removes the surprise that made early examples memorable.
An expected stinger also raises the cost of not having one. A film that ends cleanly can be read as having nothing planned, which is not the impression a studio wants.
The result is a convention that persists partly through momentum, with the strongest examples being those that stay closest to the original idea of a small reward rather than an advertisement.