There are many ways that tubing out portions of your four wheeler can improve it's off road performance. It can improve departure and approach angles, allow more engine cooling, protect the engine and radiator in case of roll over, allow larger tires with no lift, and improve visibility. Maybe the best reason is it just looks cool.
Approach and Departure
The early Ford Bronco is a great looking vehicles, but if you want to wheel hard stuff, you quickly learn that the front end severely hurts approach angle. A Jeep with bobbed bumper and flat fenders can easily get a tire on something that an early Bronco cannot because the Bronco has a large bumper and grill up front. Sean Lazelle solved this problem by getting a tube front end built for his Bronco by RockWare.
The Bronco purist will be offended, but if all you care about is driving over big rocks this is the way to go. The actual approach angle isn't improved all that much by doing this to a Bronco, but effectively, the angle greatly improved. Often, you just need to get a tire on something from an angle to get up it. With no grill, it is easy to get a tire on the rocks before the frame or body hits.
When I tubed out the rear of my flat fender, there no change in the real departure angle, but corners that used to rub, just aren't there to get in the way. I also don't get rocks jammed in the fenders wells any more. The corners don't get bent up when I hang up on one. The back of my Jeep was never a big problem because I was running wide tires, but with no back, it went from a little problem to no problem at all. There aren't many cases, but there are some obstacles and lines that I can take now, that I could not take before.
Read more of Tube It Out! Building a tube rockbuggy from the 4x4Wire Archives...