The inauguration of Aaron and his sons to the priesthood begins with a public declaration by Moses to the entire nation. Before the sacred ceremony starts, Moses pauses to clarify the nature of the upcoming events and establish their divine authority. He informs the crowd that everything they are about to see him do is based strictly on God's exact command, rather than his own personal choice [רש״י, גור אריה, שפתי חכמים, ביאור שטיינזלץ, פירושי רד צ הופמן].
The primary approach among commentators is that Moses makes this statement to distance himself from any suspicion of nepotism. He wants to ensure the people do not think he is appointing his brother simply to bring glory to himself or his family [רש״י]. Even though Moses knows that God's presence will eventually appear and prove the legitimacy of his mission, he chooses to clear himself of any doubt right from the start [שפתי חכמים]. Furthermore, this explicit announcement preempts future rebellions. It removes any future excuse from rebels, such as Korah, who might later claim they opposed Moses by mistake, unaware that he was acting under God's direct orders [אור החיים].
Beyond addressing personal criticism, Moses needs to explain the practical details of the ceremony. The specific instructions for the inauguration had been given to Moses privately but were not yet shared with the Israelites. Now, with the crowd gathered to honor Aaron, Moses presents and explains these procedures [ביאור יש״ר, תורה תמימה]. Additionally, the people are about to witness certain actions that differ from the general commands regarding the Tabernacle they already know. Moses prepares them not to be surprised by these variations, explaining that these specific actions are based on an oral tradition he received directly from God [העמק דבר].
The public declaration also carries binding legal weight. The act of reciting the inauguration instructions to the gathered assembly is an essential part of the ceremony itself, and without it, the entire process is invalid [תורה תמימה, אדרת אליהו]. The actions Moses performs, such as washing the priests, are not just temporary steps. They establish the permanent legal foundation for future generations regarding the washing of hands and feet, as well as the High Priest's immersion on the Day of Atonement [אור החיים].
Finally, this announcement relates to Moses's personal status as a king. According to law, a king is not permitted to give up his honor, and washing another person is a lowly task that would normally compromise his royal dignity. By declaring that God commanded this, Moses clarifies that he is only permitted to step outside his royal status and perform this task because the King of Kings explicitly ordered him to do so [אור החיים].