Hey there, Desert Fox. I apply clear coats for a variety of reasons.
The most common reason is related to decals. Some paints are flat, which means that decals have a hard time adhering to the model and will usually have silvering. I spray on a clear gloss coat to "smoothen" the model surface and ensure that the decals will really stick smack onto the surface.
After that, I apply another coat of clear gloss to even out the model surface, making the decals "blend" right into the finish. Thereafter, if I'm replicating a dull finish, a final coat of flat clear to remove or reduce the sheen.
I also spray on a clear coat after applying was for the same reasons mentioned above. Clear coats also protect the final colors applied to your model as well.
Hope this helps.