Emissions systems may be unneeded and parasitic additions (reasons for which people drop them, or even choose directly non-VVT heads and simplified stand-alone ECUs for an engine built from scratch).
But uncontrolled burning is never ideal, and turns quickly into detonation. On a circuit, all weather conditions are predictable, fuel is always top-notch, overfueling is welcome, emissions systems are not needed at all, car runs at WOT for much of the time and barely needs even a MAF. On the street, people may choose to be a bit prudent (or face a holed piston, a blown rod, burnt exhaust valves, or, God forbid, all of them together).
~Nautilus