LP Coversion kits are bascially an air mixer with appropriate sized holes matched to the BTU output of the fire ring or burner bar. They will lean out the LP gas mixture which is richer than Natural Gas and eliminate the black suit and smoke that is normally associated with LP gas flames.
Although they are not a necessity when LP gas is being used they are highly recommended. Without a properly sized air mixer black smoke and propane smell are likely as well as a black suit accumilating on fire glass or fire rock.