Offline Encyclopaedia has its cons and pros - you won't need internet connection but it will be harder to update.

The current version is really easy to update as it is on their server and you can still access it separately via your web browser. They can update it whenever they like and you can consult it without even having to start the game, even on your phone.

If it is offline the game will need to be updated via a patch to include any new or modified details to it. Given how hard it can be to push an update on some platforms (Stadia) I can understand why they wanted to try an online Encyclopaedia.