Manabars contain most of the nutrients required by a monk (but not all the protein, and not much energy). It has been said that they "famously discouraged at least five deadly sins". In addition to this, field agents like Weft seem to find them reassuring as a connection to home.
Manabars are known to taste absolutely awful to a monk if he has done something bad. Whether this is purely a psychological effect is not known. Usually, however, they are almost tasteless.