Rachael, history shows that fast bowlers are just as effective as good swing bowlers because they can get batsmen out in different circumstances than a swing bowler or line-and-length bowler. They can have the batsman playing on when the batsmen is "beaten for pace". They can be bowled with a fast yorker. They can can get LBW decisions with a full-toss or "toe crusher". They can get a top edge from a mis-timed pull shot. They often produce catches from attempted cut shots because it's hard for a batsman to play the ball down or judge the bounce correctly. These are just a few types of dismissals unique to fast bowlers.

I forgot to mention that Reverse Swing has become a big weapon for fast bowlers. It has been responsible for many wickets. The other delivery which is often overlooked is the "slower ball" which has fooled many a batsman and taken wickets.