When you disconnect the hoses from the canister, they are left hanging down, full of water, shut off by the valves. You can manually open the valves, in the 2026 that is done by pushing that little lever on the bracket that holds the two hoses and fits them into the canister. If you open that valve, you will drain water from the tank, flushing a lot of the crud out of the lines since the flow is so fast.
If you do not flush the line, the crud builds up and when you restart the canister a cloud of crud blows out of the output line, messing up your fresh clean water.
I try to clean my canisters every 2 months, the planted tanks and discus absolutely need it that often, the fish only tanks are not so critical.