mostly cosmetic.. certainly on a MTW, although it was once common as a means to reduce radiant heat (under bonnets or motorcyclists legs) and to offer a performance gain from the higher exhaust temperatures it offered. Not all good news though as I've heard of over lean engines and local heat spots that can fracture pipes and do nasty things etc..
its been replaced with ceramic coating mostly..
If you really like the look, worth a go..
