Aside from lift, one of the other purposes that has been shown in illustrations is that they provide two more weapons stations. However, they probably couldn't be used for such while staying within the Army's weight limit unless it's acceptable to go over to carry more so long as you meet the requirement when carrying the Army specified load. In any case, using the stations would entail a noticeable performance hit, but it might be an acceptable option in cases where maximum firepower is most important.
Regarding the second crewman, Raider-X also has a crew of two. Don't forget, the original Army requirement for LHX (which eventually became Comanche) specified single pilot. This was eventually dropped when reality stepped in. Like The Sultan said, workload.
