People do weighted abdominal exercises for a wide variety of reasons. Some people incorrectly think it will help them get a six pack (just like some people think doing a thousand crunches a day will) but it will simply just build up the size and strength of your rectus abdominus muscle (your "6-pack muscle). If there is a lot of bodyfat on top of the rectus abdominus, doing weighted abs might even make your stomach look bigger and fatter. Most bodybuilders don't do weighted abs for this reason, they want to keep their midsection as small as possible. Weighted ab exercises also does very little for overall core strength and stability since the rectus abdominus muscle only really contributes to trunk flexion.That being said, doing weighted abs can be useful to some athletes that do a lot of trunk flexion or those in contact sports, it can also be useful just for shear variety. I personally only do weighted abs a couple times a month just for variety since I am not currently an athlete.