FD have been designed for file systems where you definitely wants to share the same data structure for read/write/seek and imply that by giving it the same ID.

FD have been designed for file systems where you definitely wants to share the same data structure for read/write/seek and imply that by giving it the same ID. The fact that seek does not work on socket is just because socket is a sub specialisation of FD manipulation, just as pipes are.