One of the most important things one needs to do when he has enemies who try to hurt him is to find the way they work and dismantle them.
Our biggest enemy in this world is the yetser hara which is busy manipulating us and getting our priorities wrong. The way that he does so is by playing with our mind which makes us lose focus.
The Ramchal opens his sefer by describing the very basic way this enemy works and that is by making us think that those who learn musar must be wasting their time because during that time they could learn advanced secular studies like math and alike or even learn deep Torah studies such as gemara. The main thing is that we shouldn’t learn musar and that we get busy and don’t think about this very important study of what hashem really want from us. It gets so bad that sometimes we view those who are actually learning it as weak minded and incapable of learning gemara and that is the reason why they turn to musar.
By uncovering the mask of the yetser we are now able to go to the next step of actually learning the topic that he makes us think we shouldn’t.