The changelog for TU 3.0, which contains 10–15 GB of content, lists many bug fixes, a lot of which are related to game crashes.
These fixes are not the only changes to the game, however, as this update also includes
files that will be used for a closed technical test for the Watch Dogs: Legion Online Mode.
Online Mode will be the official multiplayer version of the game, and this update allows for the developers to perform private testing on select players to see how well the mode plays out.
The focus of the tech test, says Ubisoft, is
service stability and ensuring a smooth Online Mode launch.
Online Mode was missing from the game upon its launch in October, with the mode being delayed into early 2021 due to Ubisoft having to divert resources towards fixing the game's buggy launch state.
Ubisoft does not give a release date for Online Mode, but promises to
have more information coming to you soon.
The update is out for PC now, but may take a while to come to consoles; Ubisoft notes that the changes may be
rolled into an update closer to the launch of Online Mode.