A lot of thought goes into the planning of a training. Here's why you should follow the training plan.
If you have a look in your training plan - your coach has given you all sorts of details.
They serve two purposes:
Desired Adaptation (training what you want)
Standardization & Comparability (progress tracking)
DESIRED ADAPTATION
When you train you have a goal. More muscle, get stronger, jump higher or even better - all three. However even though all these three goals are related there are optimal ways to train each of them.
An experienced coach knows how to choose the right exercise, sets, reps, tempo, intensity and rest based on the goal, time of season, training age, weekly schedule and so on.
A lot of thought goes into writing an optimal training plan. And while it's actually impossible to write the perfect training plan we can optimise a lot of parameters based on research and experience.
So if you have a certain goal - you better stick to the prescriptions - that way you're actually training what you're training for. But there is a second reason:
STANDARDIZATION & COMPARABILITY
In training you want to make progress. We've discussed what progress can look like (and why it isn't just more weight) in detail here.
However to check if you've made progress you have to be able to compare what you did. If you do Squats in Week 1 and Deadlifts in Week 2 - you don't know if something improved. So the exercise has to be the same.
If you do squats in Week 1 and go all the way down and in Week 2 you only go half way you might be able to increase the weight but it's not comparable.
If you lower the bar for 5 seconds for 5 reps (25secs) in Week 1 and for 3 secs in Week 2 (15sec) you might feel stronger but that's just because your muscle wasn't working for that long.
If you stick to your rest in Week 1 (60sec) but in Week 2 you spent 5min on your phone between sets you probably feel better/stronger while performing the exercise.
However to know that your making progress, if the training plan works and if you should increase the weight and/or reps - you have to be able to compare the workouts.
Also important: the total volume (number of sets and reps) is also important. First it need to be enough so it triggers your body to adapt, however it also shouldn't be too much so you have to recover too long or get injured. That's why you should also stick to the prescriped sets and reps.