You need one to use Play Store on Android. At least I think you need a GMail one. Even if not, the Play Store and the Android OS itself make it seem like you need one, from the second you turn a brand new phone on. The funneling is quite heavy here.
(Heavy enough that almost everyone in my family uses a GMail account mostly just for the Play Store. My wife and I pay for Fastmail, some of other members of my immediate family have primary e-mail with a local free mail provider, but everyone found it easier to use an old (or make a new) Google account with their Android phones, instead of figuring out how to make it work with non-Google e-mail.)
This greatly confuses the Gmail and Outlook web UI but the accounts work.