There's basically two points. One is that your top 3 or 4 hitters are generally in the top 4 spots, and nowadays guys in the pen usually have better stuff than your back end starters. If your reliever can get through the first, then your starter will likely have to face those guys one less time in the game, which is actually a pretty big deal. The second point is that managers aren't gonna pinch hit in the first, so you can get the exact matchups you want.