[rlk]

Minecraft authentication requires a Microsoft account. People may not want to have one for ideological reasons, or not want to attach a game to the account they use for other purposes.

Also, not encouraging it, but disabling authentication allows players with pirated copies of MC to play on the server

[josephcsible]

But you need a Microsoft account to launch the game even if you're not connecting to a server that uses it for authentication. (Unless, as you mentioned, you're playing a pirate copy of the game.)

[xboxnolifes]

Maybe soon that'll be the case, but it is not currently for the Java edition.

[josephcsible]

Oh, they still let you use just a Mojang account? In any case, though, there's never an extra account required to use built-in server authentication that isn't also required to launch the game, unless you pirated it.

[anilakar]

No consumer-oriented software should require a VPN for authentication --

Wait, it's actually a good idea. Please put everything from Meta/Facebook behind a mandatory VPN so that I have zero chance of accidentally stumbling upon it.