Mostly convenience, i.e. just flip on both switches without concern for kickback. Convenience aside, there are two sparks with similar timing, so statistically the chances of ignition is higher.
The magneto's spark output is limited at impulse coupler RPM, while the EI offers full output. In general, start performance is much superior using the EI, so much so that quite a few folks install a non-impulse mag and always start using just the EI. I did so for about a year, and single EI start performance was always superior to the previous impulse-coupled mag.
The advantage is the elimination of the impulse coupler. Although the rate is low, they do fail, usually by shedding hard parts. The failure rate of an impulse coupler parked on a hangar shelf is zero.
I note you're in Anchorage. For you, maintaining an impulse coupler may be worthwhile, as it allows hand propping in the backwoods with a dead battery. Not much of a factor in my part of the world.