I am not Phoenix44 or cricketman and do not run my system the same way but...
Because the gravel alone can't process all the waste, even though it can make use of it.
I rarely vacuum my substrate, have a heavily planted tank, average stocking level and never use fertilisers, CO2 or other additives.
A good sized filter will keep the water clear too.
The detritus isn't so much "trapped" in the filter but "used".