Firstly, I just like the feeling of absorbing the energy. It feels good to play hard.
Secondly, I can learn a lot about an individual's style, especially if their martial arts background differs from my own.
When sparring, half of one's attention is focused on offense, and half on defense. You are an active participant, which, personally, narrows my scope to freely observe, while holding pads I become a fly on the wall. Breath patterns, eye movement, strengths, weak points, and tells are all on display.
Now, the goal isn't to use this to defeat anyone, rather, to apply the lessons to yourself. I've talked before about picking up tips from boxers and kick boxers. Tips to use and tips just for the sake of awareness.
Recently, I was training with a heavy striker. I let him pound the pads as hard as he wanted--and I moved him around the room to see how his feet moved. The lesson, however, was not about his telegraphing punches or a potential weakness to exploit in his guard. Instead, a painfully obvious conclusion was made by myself:
Don't strike with a striker! Get in close--without getting knocked out--and use what you know best.
Hey, honesty has to count for something!