LastPass's username generator is much better: https://www.lastpass.com/username-generator. With "lowercase" only and "Easy to say" turned on, the suggestions are really good. This is my go-to when I need a username, and that's as a Bitwarden user!
I would think the purpose here would be that you would use unique aliases per service to limit your own risk in the event of a site breach. However, the vast majority of websites these days require a username and an email address. In which case, if I've got 50 unique usernames but they're all tied to a single email, how much am I really protecting myself if the email address gets included in the breach?
If my data were exposed, I guess someone who realized that could try any variation of the site's name to figure out the exact one in my address, but you could always do something more unique than that. Even something like generate a BitWarden password and use it as the user for the domain.
I think a good system would be a randomly generated handle like nick836742@example.com where my real email probably isn't nick@example.com, but the number is different for each service.