They never really specified what Neelix was there for, since they really didn't need supplies.
They still needed supplies. Replicators recycle but damaged material still has to be replaced. Also consider food & some medical supplies that are perishable.
Neelix was a trader, so he knew where & how to aquire supplies as well as knowledge of Delta Quaderant currancy. His over friendly nature makes him the perfect choice as a saleman.