The fuel pressure regulator is in the outlet of the IP. What you are looking at is the fuel dampner. It softens the pulses coming out of the IP and this is supposed to reduce noise in the return fuel line.
Since I've never heard this noise, they must all be working on everything I've ever seen. ::)Somehow I doubt that, which makes me think it can't be all that important, but since it's part of the system, it's nice to know it's working.