My personal trainer is on vacation and is always having me change up my exercises, even on the same muscle group. For instance, after having me do lat pull-downs, I'm then shown the assisted pull-up which is essentially the same thing.
To me, it seems like it just makes each exercise harder to master with the right form. When I started training, I'm always able to do a lot more (relative to what my PT starts me out with) on exercises that I had been working on long before.