I think it’s kind of just an archaic holdover. They have a deadline for publishing the game physically, and while it usually extends to digitally as well, you can update the digital thing. If you get the game directly on Steam or something, you probably won’t even notice the day 1 patch being installed on top of the game, since in many cases it is integrated with the main download and not separate patches you get sequentially.
All day 1 patches truly mean is that they continue working on the game even after the deadline to begin printing the physical copies in time for release.