For cravings and aversion, the practice helps one to balance the impact of the effects. Knowing what is desirable and undesirable is one thing. To what extent do you suffer from craving or aversion is another.
Let's say you do fall into a sewage treatment tank one day. That day will be bad, that is acceptable and expected. However, if you identify with that experience and let it shape your life so that you feel awful every time you go to the toilet, that is not fine.
With Vipassana meditation, one learns to let go of the thoughts as much as one can while meditating. This, psychologically, teaches you not to ruminate on the experiences.
On the other hand, for the desire aspect, you may deeply wish for good things to continue and that if you do not have them you will be incomplete. You would be happy as long as you have that good thing, but when the time comes and the good thing is no longer there, you may suffer a lot. By seeing the impermanence of things you may be able to enjoy the good thing more without attaching yourself to it, and identifying with it, in terms like "I deserve this" or "this is mine" or "I must have done something bad, that is why this thing is going away". Seeing that both good and bad things come and go lets you go with the flow of the river easier rather than trying to hold on to a sharp rock.