One of the advantages of coilovers is that it is a spring and shock are packaged together. When setting up a custom suspension, it can save space and eliminates the need for separate spring and shock mounting locations. They also have a lot of options for adjust-ability and customization and can be rebuilt to different specifications.
Off road, they can provide more travel than a standard shock and spring, but only if set up properly.
Given similiar spring rates and shock valving, you wouldn't notice the difference between a standard set-up vs the coilover setup on-road.
Also, just my 2 cents.... and I am not an expert by any means.