Your username, user ID, profile photo, mutual friends, and mutual servers are always visible to anyone. Something you said years ago could potentially come back to screw you over at any time. Changing your username does not help, as the messages you posted back then will be displayed under your new username.
Want to start a serious game project as a developer? That clan you posted drunk messages in 5 years ago might still have someone in it who can then use those messages against you, for little or no reason at all. Gaming communities can be savage.
Sure, you can make a brand new account, but that's easier said than done, because:
1) You can't be logged into two accounts at the same time on the desktop. You must use different browser profiles or a similar sandboxing method. Definitely out of the question for mobile.
2) No way to transfer your old servers, friends, roles, etc, to a new account.
3) Making a new account isn't always a reasonable option even if you are willing to lose all of your old data. Moving from one role to another to another in a gaming career often is done gradually through connections and community (but that doesn't imply you should have to be forced to expose your system username and user id to everyone everywhere).
4) Even if you do make a new account, that new account might get disabled automatically until you provide a phone number. Throwaway numbers don't work.