Well for one things like this won't happen any more.
1. Strength - It makes the front end of the truck so much stronger. With the TTB there are lots of "points of failure" weak spots where something can break or go wrong. With a solid axle you can eliminate almost all of them.
2. Customization - You are limited to what you can do with a TTB. But solid axles can be like an Lego set, you can do almost anything you want.
I'm sure that other people can add on more. This is really a question that you should ask in the SAS Noob question thread, unless your just trying to find out why "I" wanted it. Anyway hope that answers your question, feel free to ask any more. I don't get mad when people ask questions, I used to be a teacher.