It's a thermal compensator.
Basically an engineered in vacuum leak to lean the engine and step up the idle speed in lumbering beasts when underhood temperatures rise too high.
I wouldn't say you
need it, but it was but there by the upfitter for that reason
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.