Well the benefits of going light do involve lightness, but here are mine:
Less stress on my knees, feet and ankles, back and hips. Less likelyhood of fatigue, injury, poor decisions.
Less weight means I can go farther per day, so I can take longer-distance trips or take peakbag days. Or siestas, or bathe.
The feedback loop between burning less food and water per distance means you can sometimes carry less food and water which (Ayyy!) makes your pack lighter. Or you can bring some luxuries.
Such as fishing gear, butter and lemon, which can lead to gourmet eating. Or having to even carry less food if you are super-confident.
Or luxuries like my favorite: real sturdy boots, or a toothbrush and toothpaste, medical supplies, climbing hardware, etc.
The downside at least of UL can be that you are less prepared to handle contingencies. Also, and this is something people don't ususally think about: if you do all your training with ultralight stuff, you will be setting a pretty low bar for strength. Better to train with extra weight and drop it when you go for things that are challenging. So all you ultralighters on the AT, put a rock in your bag. ;)