This allows them to better fingerprint users for advertising. Anything else is just smokescreen. More and more sites are doing this for just that reason.
I doubt this is the sole reason. Their search referrals are going down and logged out DAU in US dropped. They are looking for ways to convert casual visitors in regular users and make it harder for AI scrapers to access the data. It’s a business after all that’s being cannibalized by another business.