As this early teaching of the church shows fasting has been a practuce in the Church's life. Pope Benedicts Letter for Lent this year reminds us of the reality too. How? In fact he doesn't only mention it. He pics gasting as the very subject for his letter.
Fasting from food, drink, words, and entertainment is a way to store up time energy and resources so that we can be better focused on divine things. Loving God more enables us to have authentic love for echother and to do those things of justice mentioned in Issai today.
Fast and in the silence there in you will come closer to God who is love. Then connected with Love you will be able to love others better.
If we try to skip fasting and assume we are close to God and simply try to go out and help the poor we are fools who will be usless to the poor. We might even hurt the poor we try to help as happens in the case of the abortionist or public school promoter of bad sex.