I rewrite code to explore and understand it. This actually started as a bad habit - "Oh, I can't believe this person did X, I'm going to change that to Y". Do that enough and you quickly figure out exactly why they did X, and they were either smarter than you or much more familiar with the problem domain. But now you know a little more, you have a little more respect for the code, a little more humility, etc..

Nowadays I take it as a given that I'm probably wrong, but I start rewriting anyway. Worst case (and most common case) I have to toss the code. But I learn. Plus there's a different place your brain goes when you feel like you control the code vs. looking at it behind glass.