Critics who see a film early agree not to publish until a set date. The arrangement is a trade, and the date chosen tells an attentive reader something before a word is written.
The exchange is access for timing
Publications want to see a release before the public does, so their coverage appears while interest is at its highest and while readers are still deciding whether to buy a ticket.
Distributors want that coverage concentrated at a moment of their choosing rather than spread thinly across the several weeks of screenings it takes to show a film to everyone who will write about it.
The embargo settles both requirements at once, giving critics early access in return for the distributor retaining control over when their verdicts become visible.
Concentration is the main benefit
Reviews arriving together produce a visible surge of coverage, which reads as an event in itself and reaches people who follow none of the outlets individually.
Reviews trickling out over weeks produce no such moment, and early opinions harden into received wisdom before the release arrives.
Coordinating the release of criticism is therefore a scheduling tool, in the same way that coordinating a trailer campaign is.
The date is a signal in itself
Confident distributors lift embargoes early, sometimes weeks ahead, because favourable reviews circulating in advance are among the cheapest ways to sell tickets.
Titles held until the day of release, or until after the first paying screenings have begun, are generally expected to review poorly, and experienced readers treat a late embargo as a verdict in its own right.
This makes the practice partly self-defeating, since the timing communicates roughly what the reviews were going to say.
Conditions extend beyond the date
Some agreements restrict what may be discussed rather than only when, commonly protecting a plot turn or a casting surprise.
Others separate a short reaction from a full review, permitting brief impressions immediately while holding detailed criticism until later.
Outlets weigh these terms against the value of access, and some decline them, which usually means waiting for a public screening.
Enforcement depends on continued access
There is rarely a formal penalty for breaking an embargo. What exists is the loss of invitations to future screenings, which matters to any publication covering releases regularly.
That is sufficient for most outlets, and breaches are unusual and generally accidental rather than deliberate.
The system holds because both sides need it, though it has weakened as audience reactions from early screenings spread widely before any critic is permitted to publish.