Filter is used because the old caustic hot tanks no longer exist... The dish washers & bake ovens work okay, the shot blast makes everything it can get to spotless, but the cooling system has lots of areas shot blast can't effectively clean... The bake oven will loosen stuff but it doesn't dislodge it... Flush it with a pressure washer all you want but some stuff will still remain... So when you fill the cooling system it won't dislodge anything, but a few heat cycles & stuff starts floating around... If you have a filter in the upper radiator hose the filter will catch it & in my experience you will have to clean that filter 3-4 times before things settle down... If you don't have a filter in the upper radiator hose the debris will go straight to the radiator were it will plug up a fair percentage of the radiator core....