Three treatment levels exist because they remove three genuinely different kinds of pollution. Stopping after the wrong one doesn't leave water "a little less polished" — it leaves an entire category of pollutant completely untouched.
Primary treatment is purely physical — screens, grit chambers, and sedimentation basins that let gravity separate what it can. Secondary treatment is biological — living microorganisms actually consuming the dissolved and suspended organic matter that settling can't touch. Tertiary treatment is additional, more advanced polishing bolted on after secondary, aimed at whatever secondary still leaves behind — nutrients, fine solids, pathogens. Each stage targets something the one before it structurally cannot remove. None of them is an optional upgrade on the others; they solve different problems.