(Extracts from the Ekadasi Mähätmya)

Ekadasi first appeared from the body of Lord Vishnu during the waning moon in the month of November/December in order to kill the demon Mura.

Ekadasi falls on the eleventh day of each waxing and waning moon, is very dear to Lord Hari. Observing the vow of Ekadasi is among the first ten principles of the sixty-four items of devotional service described by Srila Rupa Gosvami. One should observe Ekadasi only to please the Supreme Lord. The meaning of the word upavasa (fasting) is to live nearby. On the day of Ekadasi one should remain aloof from all kinds of sinful activities, give up all kinds of household activities and sense gratification and live near the lord.

In the Brahma-vaivarta Purana it is said that one who observes fasting on Ekadasi day is freed from all kinds of reactions to sinful activities and advances in pious life. The basic principle is not just to fast, but to increase one’s faith and love for Govinda, or Krsna. The real reason for observing fasting on Ekadasi is to minimize the demands of the body and to engage our time in the service of the Lord by chanting or performing similar service. The best thing to do on fasting days is to remember the pastimes of Govinda and to hear His holy name constantly.”

    -Nectar of Devotion–(Ch 6)

So the main purpose is to perform more devotional service like chanting, reading or doing personal service to please the Lord, not to achieve any material benefit, but just to get pure devotion on his lotus feet, which is the topmost success of human life.