In principle, someone seeing heido15wkj6@yourraredomain.com, yua16ooaaj2@yourraredomain.com, and kqoq91inhi4@yourraredomain.com in a dump might be able to infer that all of these belong to the same user with a catchall address (especially if they can verify that the domain is unpopular via dns caching or other tricks). Using a common service adds another partial layer of anonymity between the email addresses, making one harder to track.
Especially necessary when your domain is your full name. It's painfully obvious that all addresses under the domain belong to a single person. I've been using Fastmail's masked email service for a while to hide my personal domain. And before that I had a handful of gmail addresses that I set to forward to my main address.
Theoretical yes. But never had that. It is more likely that generic, non existant emails like info@domain.com, admin@domain.com, etc. will be generated if you have a domain with a public website.
