The majority of my arguments online are sports related, and that's just done out of entertainment and fun. It is enjoyable to try and make sports related predictions and to watch them play out. I do this a lot in the real world, but arguing sports nonsense is just another form of being immersion into the sport, especially with my favorite sport of baseball where there is a community dedicated to making arguments and taking positions and watching them play out. Especially when you're right.
There are times that I get into political arguments online. I don't actively seek these out and I don't particularly enjoy getting into them, but I engage in them completely of frustration. In the real world, there are consequences for voicing your political opinions. You risk damaging your reputation and relationships with others by engaging in them. I so often walk a tightrope between wanting to make a comment or saying something, and refraining because of the potential consequences of doing so are. In the online world, I don't care much at all about how I'm perceived. It's freeing to be able to express myself in a way that is unavailable to me to do in the real world, and is an outlet for the buildup of frustration and tongue biting.