If you don't keep the spread at least 40 degrees, the temperature will be above high limit and the timer won't start the blower. If too many cycles pass without a call for heat, either by demand or the timer, the fire will go out.
When an oil burner reaches its low limit setting, the burner will shut down and so will the source of heat, which would heat the water.
When the stoker reaches its low limit setting, the blower stops but there is still a pot of hot coal which can raise the water temperature another 30 degrees after the blower stops, if there is no demand or circulation.
High limit is merely a safety limit. The boiler will never be maintained to a high limit setting.