There are two types of changes in SPT which are the advanced spark and retarded spark. Both have their own significance in producing race cars and mileage cars.
When the engine speeds up, the movement of pistons also speeds up thereby increasing the need of an advanced spark. But if the spark is produced too early then the peak pressure of cylinder may decrease. The best time to produce a spark is when the piston is down and intake stroke has just completed, this will give the maximum peak pressure of cylinder and hence maximum power to the engine.
But in many cars, the power is not required but mileage is a must. In that increasing peak pressure of cylinder will not work but decreasing the exhaust can increase mileage without increasing power. This can be done by using a retarded spark. It will decrease the peak pressure of cylinder and temperature. Lower temperature doesn’t let the formation of many emission gases like Nitrogen Oxides. Best time for a retarded stroke is when the piston is at the top during the compression stroke.