At one level they probably serve the same purpose as stars you cannot travel to, planets you cannot land on, and the various walls, doors, lockers, crates, etc. that you cannot interact with. It adds some amount of realism and completeness to the game. If they didn't exist people would complain about the lack.
It probably also serves a purpose for future features. It is far easier to change the name of an existing item than it is to add in a new item. Rename an item; change its RNG frequency; put it in a mission and you have super rare weapon codes, secret recordings or whatever data a mission needs.
However, I too am completely frustrated by the SDPs. There seems no way to determine where to find them. I've found exactly one since EDO was released. It was a grind to find everything else but it was possible and there was a bit of sense to where to find it all. SDPs? Pffft!