Some people like their kits nice, clean, and pristine, Others like them looking like they just got kicked down a hill. Weathering can add detail from the subtle to the extreme depending on your preferences. The best part is that it's extremely easy (and cheap) to do. It's also a lot of fun. There are many different ways to weather a kit. Here are just a few.
It stands to reason that if your gunpla is getting worn down, so would the markings. This part's easy. After you've fully applied your decals, simply scratch away with a toothpick. I think it adds character.