The E-jets idle the hydraulic pumps during engine starts to reduce starter loads so you don't see pressure until the start is well under way.( this is for in flight starts) Also for EW73, the E-jets can run 2 packs from one engine but the reduced flow restricts the aircraft to 31000ft. Above this it wouldn't meet the required lbs of air per passenger per minute. Some other aircraft are restricted to lower altitudes in the same condition.