Why does the circle shrink in battle royale games?
Because without it, a game where the objective is to be the last player alive would reward hiding — and the shrinking zone is a design solution to a problem that would otherwise make the format unplayable.
The problem it solves. In a last-player-standing game on a large map, the optimal strategy for any individual is to find somewhere concealed and wait for everyone else to eliminate each other. If everyone plays optimally, nothing happens. A hundred players hiding produces no game at all.
How the shrinking zone fixes it. A safe area contracts over time, with damage taken outside it. This:
Forces movement, since remaining still eventually becomes fatal.
Forces encounters, by progressively reducing the space available so players are compressed together.
Guarantees termination. The match cannot continue indefinitely, and the final stage is in a very small area where engagement is unavoidable.
Creates escalating tension, because the rate of encounters rises as the game proceeds.
Introduces a second threat, so players must weigh fighting against repositioning — which is the actual decision the mechanic creates, and where the skill lies.
Why randomness in the zone's position matters. A predictable final location would be occupied in advance. Random contraction means the advantageous position changes every match, so map knowledge is about options rather than a single memorised answer, and it prevents a dominant opening strategy.
The design decisions within it:
Timing — how long players have to reposition, which determines pace.
Damage rate, which usually escalates, so being caught out is survivable early and fatal late.
Zone size progression, which controls how many engagements occur.
Whether the next zone is shown in advance, which converts it from a chase into a planning problem.
Related mechanics serving the same purpose: diminishing loot and resources; increasing visibility; and forced objectives.
The lineage. The mechanic derives from mod-based games and the earlier battle royale concept, and its rapid adoption reflects that it solved the central design problem of the format cleanly.
The genuine criticism is that it can feel arbitrary, and that a well-played position lost to zone movement is frustrating in a way losing a fight is not.