Can you imagine a stand-up comedian having to look up his witty repartee in a book before giving his response? He would quickly be booed of the stage. While being a Dungeon Master (DM) in a game of Dungeons & Dragons isn't quite the same as stand-up comedy, some of the same principles apply: The game is best when there is a good flow of interactions between the players and the DM, with the minimum amount of time lost in between. Having to look up rules for 10 minutes is bad, and so is losing time to set up maps. There is a reason why I spend so much time printing maps instead of drawing them during the game on a dry-erase map. Not only are the maps prettier, but I also gain a lot of time during the play session.
Gaining time during the session by spending that time in preparation is especially important if you don't play all that often. My D&D group only meets one evening every two weeks. I don't really want to spend a significant portion of that time drawing dungeons, setting up fights, or looking up rules. The rules you can sometimes make up on the spot, but for a decent combat encounter everybody needs to know exactly where everybody is on a battle map. If you lose too much time creating the maps as you go, you also break immersion, and tend to distract the players from the game. I am not a fast drawer.
And of course preparing my adventures is fun to me, not work. It is how I spend my weekends these days, instead of playing MMORPGs. I have a certain pride in running a game which is enjoyable for the players, and running smoothly without unnecessary delays. If that means spending some hours reading rule-books, and designing and printing maps and handouts, I don't mind.