This increased radius improves the ability to maximize your weight allowance on the steering axle. The extra frame rail length also allows room to mount generators or tool boxes. For the most part, I've also found a longer wheelbase generally improves resale value as the truck is more versatile in most cases.

What is the advantage of a longer wheelbase?

Cars with long wheelbases tend to have better ride quality than those with short wheelbases. This is simply because there's more time between the front and rear wheels hitting any bumps, so the car is less likely to become unsettled.

Why do some semis have a long frame?

Generally, semi-tractors with an extra long wheelbase are designed to accomodate extra storage/cargo space in a box behind the cab and in front of the 5th wheel area. You'll see these quite often with freight expediters, who frequently want to secure loads in different compartments for a variety of reasons.