Si puo' omettere il lavabo?

The washing of hands at the end of the offertory rites may never be omitted at any Mass. It is a significant rite and expresses the priest's need for purification before embarking on the great Eucharistic Prayer.

The omission of the rite may stem from a theory of its origin, popular a few years ago, that the rite was originally practical and was required because dust from the loaves handled during the offertory during the ancient celebration needed to be removed from the celebrant's hands. Only later was a spiritual meaning given to the rite.

Thus, some argued, the advent of pre-prepared hosts had rendered the rite obsolete. This theory, while coherent, has the disadvantage of being wrong.

Further research into the ancient rites has shown that the rite of washing of hands (dating from the fourth century) is older than the procession of gifts, and even after this practice was introduced the celebrant often washed his hands before, not after, receiving them.
Thus the rite has always had the sense of spiritual purification and validly retains this meaning today.
http://www.zenit.org/article-9481?l=english