They probably had very few servers for this Pre-Purchase Beta and on release they will need more for all the additional players.
On top, migrating server data from one to another and getting server structure ready can take a while. This is the most likely scenario.
It’s possible they’re doing a bigger update than just a version increase. They might have a completely different final build for release to protect release stuff from datamining or because they’re removing analytics or other beta back end bits. Also as someone mentioned later in this discussion the server infrastructure is probably part of it as well as if there were more critical / engine based fixes they want to implement that requires them to take everything down and push updates to be internally tested before pushing back out to the public. I would say there’s a decent chance it’s because they’re trying to roll over peoples data and want to make sure everything is compatible with the final live build client and they don’t have any egregious bugs or data vulnerabilities. Also like someone said, hype.
Go touch grass OP, watch a movie, animu, fap or whatever.
They need to run server maintenance routines probably and upload the actual game client, if you didn’t notice we are playing on a beta branch.
Updating all the various regional servers, adding additional capacity to handle the day-one/week-one/ spike. And taking them offline allows for patching and testing without affecting gameplay for players.
They’re probably triple checking everything because last time they didn’t, they launched with an old internal build, denied almost all bug reports during launch, until they finally checked after a few weeks and realised that their internal test machines had the correct build but Steam didn’t.