They don't mention it, but there's a privacy benefit to generating usernames. Most users are predictable/lazy and will use the same username on every website. However, tools exist for finding accounts across services that have the same username. Generating a unique username and storing it in your password manager would make it easy to evade these tools.
https://github.com/topics/username-search
I use different usernames per-site, but I tend to take a couple seconds to think of something (hopefully) clever.