Why are aromatic compounds such as toluene and oxygenated hydrocarbons such as ethanol generally used as antiknock agents?
Simply because they work.
This is the province of an engineer rather than a chemist. The addition of certain additives to your hydrocarbon mix results in better performance in your combustion engine. When you go to fill your car up with gasoline, different brands will tout different additives; most formulations would be very similar.
The presence of oxygen in alcohol additives may provide better combustion, but there is a good deal of incomplete combustion that occurs in a well-tuned internal combustion engine - as witness