From my observations USER does exactly nothing.
> PASS xd
> NICK justinfan1234
< :tmi.twitch.tv 001 justinfan1234 :Welcome, GLHF!
< :tmi.twitch.tv 002 justinfan1234 :Your host is tmi.twitch.tv
< :tmi.twitch.tv 003 justinfan1234 :This server is rather new
< :tmi.twitch.tv 004 justinfan1234 :-
< :tmi.twitch.tv 375 justinfan1234 :-
< :tmi.twitch.tv 372 justinfan1234 :You are in a maze of twisty passages, all alike.
< :tmi.twitch.tv 376 justinfan1234 :>
> USER justinfan1234 8 * :justinfan1234
The RFC 2812 spec says that:
TMI doesn’t support user modes properly nor does it have realnames.