Every year we seem to see titles come out that are incomplete or have game breaking bugs. Day one patches exist to fix these on release. Why are they fixed with patches?
The technical portion behind day one patches is that a game has to go through very specific standard testing to get their discs printed in time so they can be distributed globally. This is months ahead of release normally and to be fair they do have to show a certain amount of the game works for this to even happen. On tight deadlines and budgets sometimes key game features are left out of a disc release so that this process isn’t delayed while the features are fixed. This does not excuse half released games being released and we know the system isn’t infallible. (Batman Arkham Knight)
That is just a small piece to explain why these patches exist. Explaining away giant patches though that is a completely different story.
Halo is going to have a 9GB day one patch, this patch is meant to be adding in the MP functionality which is not currently available, I am expecting though that for those digital pre-loaders this patch should be handled in a slightly early pre-load schedule instead of on launch.
Assassins Creed Syndicate also has a day one but considerably smaller, even on S.A standards we can handle a 500 odd mb patch. (sorry mobile broadband users)
Also have a trailer (above).