Why do we fast during Lent?

Jesus is questioned about fasting in Mark 2:18 -20

"Now John's disciples and the Pharisees were fasting. Some people came and asked Jesus, "How is it that John's disciples and the disciples of the Pharisees are fasting, but yours are not? Jesus answered, 'How can the guests of the bridegroom fast while he is with them? They cannot, so long as they have him with them. But the time will come when the bridegroom will be taken from them, and on that day they will fast.'"

Matthew 17:21 also mentions that fasting is necessary to help drive away evil:

"But this kind does not come out except by prayer and fasting."