Having far more fine control over, of all things, email. Being able to operate an entire domain and control what happens with every email address in it. I now have a different email address for every legal entity I interact with, sometimes on a per-interaction basis. If I get spam, I know exactly where they scraped the email address from and I can set that specific email address (plus or minus any other data such as which domain it's coming from) to reject with a custom response or display any other behavior in the email protocol, which is surprisingly extensive once you start looking at it.

It's entirely up to you. I did have a bunch of such certs, but none of them were actually useful for real-world setups; the knowledge I used for self-hosting came from me mucking around on the internet and reading RFCs and instruction manuals.