Gravel under solid drain pipe?
What does the gravel provide:
The gravel helps allow you to build the drainage slope for the pipe and provides a back-up drainage medium for leakage.  In commercial projects we spec the drainage starts with perforated pipe then truncs into main drainage that is non-perforated and we require the gravel to be carried throughout, as a secondary drainage path.  

Why do I need the extra drainage in gravel:
If the pipe connection has a flaw or if the ground shifts a pipe crack could form.  If the pipe is backfilled directly with soil the water will collect against the pipe and soil without somewhere to go. This collecting of water will make the soil saturated, cause additional issues and water infiltration. The gravel will allow the water to drain away from the pipe and down the path of least resistance in the direction of the pipe to the outfall.