Reviews of a film, game or album often appear within minutes of each other, which looks coordinated because it is. The arrangement solves a problem for both sides before it serves anyone's marketing.
An embargo is the price of early access
Distributors give critics material before the public release. In exchange, critics agree not to publish before an agreed moment.
Without that agreement, the first outlet to receive a copy would publish immediately, and everyone else would be reacting to their coverage rather than to the work.
The embargo removes the advantage of speed, which is what makes it possible to hand material to a hundred people at once.
It buys the critic time to actually finish
A long game cannot be assessed in an afternoon, and a review written under a race condition is a review of the opening hours.
Fixing the publication time lets a critic use the full window for the work rather than for beating a competitor.
Where the window is short, the reviews reflect it, and careful outlets say so in the text.
Access is uneven as well. A large outlet may receive material weeks ahead while a smaller one gets days, so a single embargo time conceals very different amounts of preparation.
It also protects a marketing sequence
Campaigns are planned as a sequence, and reviews are placed at the point calculated to convert interest into a purchase.
Too early and the enthusiasm dissipates before release. Too late and nobody has read anything by the time they decide.
Distributors also want reviews landing after their own material has established what the work is, so criticism arrives into a formed impression rather than an empty one.
The timing is a signal
An embargo lifting well before release generally indicates confidence, because early positive coverage has time to spread.
An embargo lifting on release day, or after it, suggests the distributor expects reviews to reduce sales rather than increase them.
Critics and readers both read this signal, which means a late embargo damages a work before a word has been published about it.
What breaking one costs
Embargoes are contractual for some outlets and conventional for others, and the sanction is usually exclusion from future access.
For an outlet that depends on covering releases at the moment of release, losing access is a serious commercial problem.
That imbalance is the standing criticism of the arrangement: the party setting the terms is also the party being reviewed.