>Why, exactly, would a forced change of habits be for my own good?

At the personal level, it wouldn't be. It makes a lot of sense, and I do the same with Fastmail.

At the corp level where it's often in M365 cloud, you've got hard limits from Microsoft on one hand (100GB primary mailbox - period), and corporate data retention limits on the other. Legal often has strong opinions on how long you are allowed to retain emails which you may or may not be able to personally override. Could be just a few years, which forces a different strategy.

I'm not sure on the details of Google, but one imagines corp workspaces have equivalent interests.

No, not conversations, actual data. Think reports, invoices, large PDFs, etc. Emailing files to yourself, that sort of thing. Then they end up with multiple PSTs.