Less muscle strain and less stress on the heart
Extra assistance means less strain on your muscles and joints, particularly since the motor will give you most support on hills and accelerations, when most effort is needed to be invested in cycling.
While riding an electric bike, your heart will be less stressed and less strain will be put on the latter compared to riding a normal bike since the motor helps smooth out the periods of harder exertion, but you will still get a good aerobic workout from riding, which is one of the best, if not the best thing you can do for your health.