Well there's much more to it than just cutting down on the work for the courts and judges. For one thing, it's a criminal offense to lie while under oath. If you're in a court room, you lie, you get more charges added to your record. By pleading guilty, you're telling the truth, so you don't get those same charges. Also, some people have psychological disorders, where they think the crimes they're performing are justified, "I had the right to kill that man because he stole my pudding cup." and there are some where they don't even know they did it, "Wait, you mean that's me on the video stabbing the guy? I don't even remember that." so people like this need to be in prison longer because there they can get the psychological counseling they need in order to properly reform to society.