The best time to fire the spark depends on the engine's rpm and load, both of which vary. Starting is one extreme: as cold engines are hard to ignite it's worth delaying the spark until after compression has warmed the mix and the pressure has dropped enough to allow a fatter spark. (High cylinder pressure makes the fuel a better electrical insulator.) Late timing for firing the first cylinder is grossly inefficient when the engine is running, so the timing has to change. It was originally left to the driver to adjust for 'best' results, which are a complicated combination of fuel economy, torque, power output, and temperature management.
Drivers aren't particularly good at managing spark timing – the lever is adjusted until the engine feels right. Works well enough when the engine is working at a constant rate – cruising – but drivers aren't quick enough to deal with stop-start town driving and short slopes, nor do they have enough information to get the setting spot on.
Manual advance/retard turned out to be quite dangerous in early aircraft, because pilots would forget to adjust it, particularly when dealing with emergencies! A mis-tuned engine wouldn't deliver full power and might overheat and seize. Manual adjustment was bad news for fighter pilots; not good in a dog-fight to have to keep tuning the engine!
The answer was to adjust timing with an automatic mechanism and it took many years to refine them. For most of my driving life it was done by rotating the points with the engine vacuum, because engine vacuum is a good indicator of load. And, because the points rotate relative to rpm, the mechanism gets the other major clue needed to adjust timing. It's better informed than the driver about the engine's innards, it reacts quickly, and it's always on the job. Although the mechanism is simple, designing it to work properly over the required range was a major challenge, and it was refined many times over the years. It was also designed to work alongside the fuel system – another bit of the engine hard to manage manually. Most vehicles made after 1945 come with automatic advance/retard.
Modern engines get even better performance by replacing mechanical mechanisms with an electronic management unit. Much faster and with many more sensors. Knock, engine temperature, vacuum, exhaust content, air temperature and humidity, oil pressure and whatever else the designer finds helpful. The EMU's ability to manage fuel, timing and pother parameters together is a major advantage. And because EMUs are computerised, they can optimise for several different profiles : sport, urban, low-pollution, cruise-economy, creeping home in an emergency, maybe controlling the suspension as well as the engine.
Most of the detail control of cars has been taken away from the driver, and there's more to come. Extend the EMU with a system of sensors detecting the cars position on the planet, and telling it what's in front, behind, and at the sides, and there's no reason to have human drivers at all!
Dave
Edited By SillyOldDuffer on 12/07/2020 11:01:21