By varying your exercises strategically not only will you gain strength and muscle faster, but you lay the platform for continued long-term progress over successive phases.
A successful training program should be built around a core group of key exercises which you rotate through several phases of training. Your exercise variation shouldn’t be random, haphazard, based on the latest fitness fad, or left to guesswork. Instead, it should fit into your overall plan, compliment what has gone before, potentiate what follows, and have a specific purpose. You need to have a logical strategy in place which guides how you vary exercises.