(if my understanding is correct)
Consider the case of two users, Alice and Bob. Alice has sync enabled, Bob does not.
Bob wants to check his email on Alice's computer, so he logs Alice off and logs into to his account. This syncs across all website he visits (due to shared auth cookies), but doesn't sync to the browser itself. Chrome is still logged into Alice's account, so Bob's browsing history is synced, but to Alice's history. This can have any number of unwanted consequences, from privacy consequences to Bob depending on whether or not you think Alice or Google are bad/compromised, to weirdnesses for Alice when she tries to check her history again.
Post this change, Bob logging in to Gmail on Alice's computer will log out of Alice on Chrome, and log in to Bob, meaning that Bob's history is no longer synced. So for Bob, this is a privacy increase (since now Google and Alice have less access to his browsing history) in that situation, and a usability improvement for Alice.
You could maybe get a similar effect by having account consistency be a thing that always logs the current user out and only also logs you in if you opt in, but that can also I think lead to weird situations for everyday users.
In other words, the point of signing in is to make sure no one else can accidentally (or intentionally) siphon away your browsing history.
Consider the case of two users, Alice and Bob. Alice has sync enabled, Bob does not.
Bob wants to check his email on Alice's computer, so he logs Alice off and logs into to his account. This syncs across all website he visits (due to shared auth cookies), but doesn't sync to the browser itself. Chrome is still logged into Alice's account, so Bob's browsing history is synced, but to Alice's history. This can have any number of unwanted consequences, from privacy consequences to Bob depending on whether or not you think Alice or Google are bad/compromised, to weirdnesses for Alice when she tries to check her history again.
Post this change, Bob logging in to Gmail on Alice's computer will log out of Alice on Chrome, and log in to Bob, meaning that Bob's history is no longer synced. So for Bob, this is a privacy increase (since now Google and Alice have less access to his browsing history) in that situation, and a usability improvement for Alice.
You could maybe get a similar effect by having account consistency be a thing that always logs the current user out and only also logs you in if you opt in, but that can also I think lead to weird situations for everyday users.
In other words, the point of signing in is to make sure no one else can accidentally (or intentionally) siphon away your browsing history.