[Jac Mac]

At Idle & any time the throttle is not wide open the cylinder can not get a 100% full of fuel & air, so the effective compression ratio is much lower than at higher rpm/full throttle so a certain amount of initial advance is required. Once you approach 100% cylinder filling then ignition timing can remain at a constant level. Supercharger, nitrous & turbo setups have high rpm ignition retard since they boost compression ratios by filling more than 100%-- some well thought out NA motors can also acheive slightly more than 100% fill @ certain RPM also.
How much total timing is a function of the combination of parts in the motor. For example if you & a mate build similar motors using all the same parts & his motor requires 38° total to produce the same power as yours does with say 32° total then you made a better job of getting all the clearances like squish etc right.