All photos transferred from your computer via the iTunes sync process are optimized for viewing on the iPhone. The same applied when transferring photos to an iPod that supported photos being transferred so the name of the optimized folder is iPod Photo Cache which remains the same name for photos transferred to an iPhone and any other iOS device.
The iPod Photo Cache folder includes nothing that is accessible and can't be deleted. If deleted, all photos selected to be transferred to your iPhone from your computer will be re-optimized again and re-transferred along with the iPod Photo Cache folder being recreated.
This has nothing to do with photos/videos imported by your computer from the iPhone's Camera Roll.