Do I really have to vary the exercises that I do during every single workout?
It all depends on workout frequency and intensity.
The main goal of exercise is to work a muscle hard enough and often enough to strengthen it. However, you must also give it time to recover and repair.
It is important to realise that it isn’t exercise itself which makes you stronger and fitter – it is the period of recover after exercise.
During the time post workout, your muscles repair themselves to be stronger, faster, and tougher than they were before.