Because your personal data and browsing habits can be monetised.
"They" sell it under the guise of conveniently taking your browsing history from one device to another (your PC to your phone and vice versa), but actually the big companies have realised that _knowing your browsing habits_ as intimately as possible means they can profit by targeted advertising and so on.
I think the question was asked from the user's standpoint rather than Google's. In other words: Why, as a user, would I bother signing into my browser?
This would explain why, recently when I opened MS Edge, it sucked up all my bookmarks from Firefox and then signed me in to itself using my Microsoft account - all automatically and without my permission.
Because your personal data and browsing habits can be monetised.
"They" sell it under the guise of conveniently taking your browsing history from one device to another (your PC to your phone and vice versa), but actually the big companies have realised that _knowing your browsing habits_ as intimately as possible means they can profit by targeted advertising and so on.