Incredibly helpful to track who leaked your email and to whom, better control spammers, etc. Just checked on Have I been Pwned, and I have 30 emails from my domain there [1]. Anecdotally, there's at least another 50-100 email addresses that I get spam regularly, so HIBP is a small subset of all the leaks.
Easy to implement. I use G Suite with Gmail[2], but it works with most other providers.
[1] https://haveibeenpwned.com/
[2] disclaimer: I work for Google, but not on G Suite team, and I pay for the service on my own.
You could also use the hash of the domain name with a secret salt value. Then if you somehow lost your login info, you could still figure out what your recovery address was if you still knew the salt.
Only spam I get is on leaked addresses.