Hitters will often crowd the plate in order to have a better swing at pitches on the outside half of the plate. The hitters hope that the pitcher will be scared to throw inside because they might hit the batter. The brushback helps a pitcher to "reclaim" the corners of the strike zone by forcing the batter to stand farther away.
While controversial, many people knowledgeable about baseball acknowledge that the brushback is an important part of pitching, and a part of baseball. However, there have always been some headhunters who attempt to hit the other player for various reasons.