If you email someone on gmail, that one email is compromised yes. But not the entire sum of all your activity.
So you're protecting against someone seeing everything you're doing.

And further, the vast majority of websites will force you to register using email. So just controlling that email is all the stuff you own. Traditional social media accounts, fiat banking, websites, forums, crypto exchanges, dexes, ect

Also you can use something like anonaddy to hide that it's even you that's emailing gmail.  So gmail would only see each conversation in isolation, and not even know your real domain