If you still do regular water changes you are actually doing the same thing with taking old and used out and replace this with clean water.
The plants and the substrate also function sort of like a natural filter, biologically by numbers and volume of organisms it seems to be a better filter than a canister. Then we could make the statement in a planted tank the canister is mainly taking free-floating particles from the water. If you don't take this out by means of filtering this will finally sink into the substrate or gather on the bare bottom as brown slimy sludge. Similar to what you can see on the bottom in sumps filters. Even with a filter there will always form sludge on the bottom, without one a little more and a little faster.