In his commentary on the Treasury of Manifest Knowledge, Geshe Tenzin Tenphel explained that virtuous actions committed in the desire realm are moveable — meaning that it’s not definite how and when they will ripen. You could be reborn as a human due to your merits and yet find yourself in a barbarian land. If, however, we immediately dedicate our virtuous actions with sincere prayers of intention, then the results will be more definite. Thus, it is very important to dedicate each virtuous action purely.
“It is one of the characteristics of action that without directing the accumulation of virtue by prayers and intention concerning their ripening as otherwise, one can never be sure where they will ripen.”