Why do you people use services like Simple Login for aliases? Why not use the service by the provider?

I have the feeling that I miss something big here, so I beg for your help.

I am about to move email addresses (either Mailbox or Posteo, mind you) and I want to go all in regarding security, meaning I will create several accounts for different purposes and give several sites an alias. As far as I know, at least regarding my two providers mentioned, you can just create aliases on their site, so there is absolutely no need for external services like Simple Login.

Hence, I don't quite understand what these services are there for. Is it solely for custom domains (that's my best bet) or something else entirely? Should I actually trust these external services more and why?