The practice of ‘giving something up for Lent’ is an important way of fasting. Fasting is good for us for four reasons:
First, at a human level, like dieting, fasting disciplines our desires. The things of this world are good, but we frequently want them in a way that is bad for us, or we want the wrong things at the wrong time. We need to discipline our desires, and this is what fasting does. This doesn’t mean we fast continually: Christians have feast days as well as fast days, but fasting enables discipline.
Second, at a supernatural level, more than mere dieting, fasting is a prayer. It thus needs to be offered to God; ‘offer your very bodies as a living sacrifice acceptable to God’ (Rom 12:1). In particular, fasting is something we can offer for our sins: in atonement and reparation for past sins, by uniting them to Christ’s sacrifice on the Cross. In general, fasting is something we can offer as a prayer for matters of great importance, as Christ told his disciples that some things can only be achieved by ‘prayer and fasting’ (Mk 9:29). Also, during Lent, uniting prayer and fasting imitates our Lord who both prayed and fasted in his 40 days in the Desert. Fasting without praying can sometimes just make us grumpy and disagreeable!
Third, fasting (and any form of penance) is also a means of detachment: when we deny ourselves some form of pleasure we help to detach ourselves from it; this helps to orient ourselves more on God and less on earthly things.
Fourth, fasting can change the way we act towards others. If we’re purifying and detaching ourselves, then we should be more free to love. One way we do this is by the traditional Lenten practice of giving to the poor.
Finally, this can be summed up by noting the Church’s threefold Lenten remedy for sin: prayer, fasting, and almsgiving (giving to the poor). These three should all go together, not in opposition, i.e. it’s not enough to say, ‘Oh, I’m not giving up things, I’m doing something positive!’ Each of us would do well to add a small part of each of these three to our Lenten season: add a small prayer to your usual daily or weekly routine, give something up for Lent, and give some money to a good charity.