The closer the leading and trailing plugs fire the earlier in the stroke the fuel burns and more power can be made. Rotaries already have a problem with the fuel still burning in the exhaust.
Though you can't have the plugs fire at EXACTLY the same time because of the position of the trailing plug. Too early and it will ignite the next chamber WAY too soon.

Knock Knock.
Who's there?
Detonation.
Detonation who?
KABOOM!