My assumption:
Signing up gets you in the queue for the upgrade. They are not going to be able to upgrade everybody on the 29th.
If you sign up they will give you a notice when you are able to upgrade.
yeah...that’s about it....it will take some time to distribute the update..
The best reason I can come up with for getting it is that the new browser is supposed to be a real big improvement. (and you can’t get it without the entire OS upgrade)