The starter has a very tough job. It has to get the entire reciprocating assembly working from a less than optimal leverage location in the accessory case of the engine. Mechanically, given enough gearing you could pull a freight train with a wrist watch, but practicality gets in the way, so if you can get one cylinder to help the starter to get things moving that's a big time plus. Magnetos produce more voltage as their rotational speed increases so down low, not much voltage or amperage is available to make a weak spark jump the gap to ground and set off a cold, wet charge of fuel/air in a cold cylinder particularly if the carbon based unit in the seat is a believer in 'more primer strokes is never enough' (same school as 'it's snowy and slick out so I'll give the car more gas').
The mags fire the plugs in order but again give a very weak spark (if you touch a lead you may not buy into that, but you can kill yourself with a 'D' cell battery, it's amperage not voltage and about 1.8 Amps is enough) so anything you can do to help the engine so much the better.
You are taught as an engine mech, to touch the impulse lead for a turbine engine to the case when removing an ignitor to ground it out. A co-worker had done this routinely for years, and just that one time, the booster was holding a full charge and as he touched the lead to the tailpipe shroud he got a huge intensely white spark, when he turned around he had the strangest startled look on his face, all he said was 'Now I understand..'
TRIVIA ALERT the mags on a T/FD dragster or F/C pump out 80 Amps to fire the charge that makes 8000 H.P.
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Luckily we still have Rich, Taigh, and others, but it'll probably be quite a while before any of 'em retires or can spend that kind of time with us... 