In the Brahma-vaivarta Purana it is said that one who observes fasting on Ekadashi 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 Krishna.
The real reason for observing fasting on Ekadashi 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, Chapter 7