Engine wear and tear
All engines are designed and built to run on petrol with a specific octane, or cetane in the case of diesel. The problem is that over time, as a car ages, its thirst for octane increases. This is caused by the accumulation of deposits inside the engine, although, the main problem from carbon deposits, gums, varnishes, and dirt also play a part. When your engine´s "appetite" for octane increases, it will eventually lead to obvious signs of octane starvation such as poor drivability, lack of engine response and/or power, excessive exhaust emissions, knocking and pinging
Previously, the only options were to run the vehicle on expensive higher octane fuel, to get it decarbonised with additives which dissolve carbon residues or, perhaps, add one of the many doubtful octane boosters. However, none of these options are entirely satisfactory as some have questionable long term effects, and all work out to be expensive in the short and long term.