Couple other reasons to stream -
If you and a bunch of friends typically stream you can see what they are up to. Might make it easier to join up with one.
In sports games it's not uncommon to stream for the instant replay. Buddy of mine streams when playing Madden so he can see the previous play on a 2nd monitor immediately.
Doesn't take but a minute or two to slap a Paypal donation button on your twitch page. I have a couple friends that stream for the heck of it and have received random donations or donations from people asking to join/play with them randomly.
Also a good way to record videos, make highlights and clips. You can archive your play videos on twitch. Need to show a tip or trick to a friend you can clip it. Making clips is super easy on twitch.
There's plenty of reasons outside of just trying to make money to stream. But, it's not for everyone. I rarely ever stream.