The gap between an advertised start time and the film beginning has grown steadily. That block is not filler; it is one of the few parts of the evening a cinema fully controls.
Ticket money largely leaves the building
A distributor takes a share of box office receipts, and for a major release that share is high in the opening week before declining over the run.
The exhibitor's own margin on tickets is therefore thin precisely when the auditorium is fullest, which is the opposite of how most retail businesses work.
Screen advertising and concessions are retained in full, which makes them structurally more valuable than the film itself to the operator's accounts.
The block has a defined order
Commercial advertising runs first, sold by an agency on behalf of the cinema and priced by audience size and film classification.
Trailers follow, and their placement is negotiated. A distributor may require that its trailer runs in a particular position or in front of a particular title.
Local advertising and cinema promotions fill the remainder, and the whole sequence is assembled centrally then delivered to each site as a programmed playlist.
Late arrival is priced in
Audiences know the advertised time is not the start time and adjust, arriving during the block. The cinema plans for this rather than fighting it.
A published start followed by a known delay reduces disruption once the film begins, and it gives late arrivals a window in which to be seated without complaint.
It also extends the period during which the concession stand is trading, since the queue continues to move after the advertised time has passed.
Trailers are the main marketing channel that remains
A trailer shown to an audience that has already chosen to visit a cinema reaches exactly the people most likely to return, which no other placement achieves as precisely.
Distributors value that reach highly, and trailer placement is treated as a negotiated element of a release rather than as a courtesy from the exhibitor.
Length limits exist by convention and by agreement, which is why trailers cluster around a similar duration despite there being no technical reason for it.
Scheduling constrains the whole thing
A screen must be cleaned and reset between showings, and a multiplex staggers start times so that arrivals and departures do not collide in the foyer.
The advertising block absorbs the slack in that plan. Shortening it would require either longer gaps between showings or fewer showings each day.
Cinemas that advertise the true film time exist, and they generally run a shorter block and accept lower advertising income as the cost of the promise.