The obvious is fuel savings. The Transit is also built to accomodate working out of the van better, It has a lower floor and a higher roofline a lower load in height and doors on both sides and the space in the box is actually not much smaller than the larger Econoline.
If you need a large powerful fullsize van there is no comparison but if you have light duty work and require a smaller more efficient van than the Transit is a great alternative to the larger van.