Bill Gates casually chats with people on Reddit when he isn't saving the world

You simply don't learn anything real from rich and famous in social media unless it's negative like showing lack of impulse control.

Using Reddit casually can be person being casual or it can be well calculated "using Reddit casually" PR strategy. With people like Gates or Schwarzenegger it's definitely partly very good PR.

u/thisisbillgates shows how to use social media to do PR, promote things you want to promote in very smart and engaging way. Gates knows that everything he says with that handle will be noticed and archived even if he does it in seemingly low key way.

If he really wants to be low key, he uses his alternate account.