A game bought on release day usually downloads a substantial update before it can be played. The patch exists because of a gap between when a game is finished and when it reaches players.
Certification and manufacturing create a deadline
Console releases must pass a platform holder's technical certification, which takes time and cannot begin until a build is complete and stable enough to submit.
Physical copies then have to be manufactured, packaged and distributed to retailers worldwide, which adds weeks more and requires the disc contents to be locked well before launch.
That locked build is a snapshot of the game at a point that may be a month or more before anyone plays it, and it cannot be changed afterwards.
Development does not stop at the deadline
Work continues through the gap, because a team that has finished submitting is now free to fix everything the submission deadline forced them to leave.
Testing also continues, and problems found in that period can be addressed in a patch even though the disc version is already being pressed.
The patch is not only bug fixes
Updates frequently include content that was complete but arrived too late for the locked build, along with balance changes and performance improvements developed after submission.
They also carry corrections that only became possible once the game ran on retail hardware in retail conditions, which testing environments approximate rather than reproduce.
Localisation updates appear for the same reason, since translated text often continues to be revised after the deadline that fixed the disc contents.
The practice has changed expectations
Because patching is available, the point of submission is no longer treated as the point of completion, and schedules are planned with the update window built in.
Critics of the practice argue this has normalised shipping unfinished work, since a deadline that can be partly deferred exerts less pressure than one that cannot.
It creates a real problem for preservation
A disc without its update is often a different and worse version of the game, and in some cases it is missing content entirely.
When servers distributing those updates are eventually retired, the physical copy remains playable only in the incomplete state that was pressed onto it.
That is the practical cost of the arrangement, and it applies most sharply to games whose launch build was furthest from what the developers intended to release.