הלכה על זבחים 38:4

From the roots of the commandment is the fixed foundation that we have said [that it is] to aggrandize the glory of the [Temple] and all of the [activities] that are done there. And therefore it is fitting to clean the hands, which are the [things] that are involved in the work at all times when the priests are touching the contents of the [Temple]. And from this root, they, may their memory be blessed, said (Zevachim 19b) that the priest does not need to sanctify his hands between one service and [another] service, but rather once during the morning, and he may [then] serve the whole day and the whole night - and that is when he does not sleep and does not urinate and does not remove his mind. It appears from all of this that the intention of the washing at the beginning is only for the aggrandizement of the glory of the [Temple], since even if he was pure and clean from the beginning of his arrival there, he [still] needs to wash. And once he has started the service, he does not need to wash again between one service and [another] service, except for on Yom Kippur, due to the stringency of the day. [This is all] since we hold and see in our hearts all of the business of the service of the [Temple] as pure, clean and holy.