without the auto-pilot on, hand flying an aircraft to maintain level, let's say to our assigned height of 30,000ft (or more professionally, above 13,000ft onwards, we refer it as flight level 300 /FL300) is not easy. additionally with bigger jets, its not instant reaction everytime you make adjustments. it takes time.
plus, flying so high up in the sky, making these adjustments can be very dangerous. with a mere 150ft tolerance (though it may sound a lot, but really it isn't) to attain the assigned altitude, auto-pilot is the way to go. breaching safety codes, ample reports which could jeopardize your career and licence would come your way unavoidably. thank God for auto-pilot!