The anti-drain vack valve helps to keep the filter full of oil and not draining back into the crankcase while the engine is shut down(many don't work well), so that upon startup, the engine will not be starved for oil(no clatter upon startup). Filters without anti-drainback valve (ADBV) are either for applications where the filters are mounded vertical(straight up) or for applications where the engines have an internal ADBV allowing the filter to remain full while the engine is shutdown.