Is invoking peace and blessings upon the Messenger of Allah (peace and blessings be upon him) an expression of need or of reverence?  

Sending peace and blessings upon the Messenger of Allah (peace and blessings be upon him) is one of the highest expressions of reverence. He is the Seal of the Prophets and Messengers, sent by Allah Almighty as a mercy to the worlds. Allah Almighty says, “And We have not sent you except as a mercy to the worlds” (Quran, 21:107).

Allah Almighty also named him al-Raʾuf al-Rahim—the Most Kind, the Most Merciful. Allah has moreover commanded Muslims to send prayers upon our master, the Messenger of Allah (peace and blessings be upon him) after stating that He and His angels do so. He says, “Indeed, Allah and His angels bless the Prophet. O you who believe, invoke blessings upon him and salute him with abundant salutations” (Quran, 33:56). The noble Messenger (peace and blessings be upon him) also encouraged Muslims to invoke blessings upon him; he (peace and blessings be upon him) said, “Whoever sends blessings upon me once, Allah will send blessings upon him ten times” (recorded by Muslim in his Sahih).

Based on this, it is incumbent upon every Muslim man or woman to invoke peace and blessings upon the Messenger of Allah (peace and blessings be upon him) whenever his name is mentioned in their presence, so that they may attain reward from Allah Almighty.

And Allah Almighty knows best.