Fasting is all about the 'disposition' of your soul. That means the 'condition' or 'state' that your soul is in. In order to prepare the soul, we have to prepare the body.
We are physical beings. When we force our body to do something hard, like not eat or drink, it reminds us that hunger and thirst for spiritual food is even more important.
Since the body and soul make up the one person that you are, they have to do things together . . . like get ready for Jesus in the Eucharist.