It is a command of Allah to the believers when He said (the meaning of); "Indeed, Allāh confers blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [Allāh to confer] blessing upon him and ask [Allāh to grant him] peace.(Quran kareem 33:56)
the salah of Allah upon the prophet are showers of mercy, while the salah of the angels upon the prophet is asking Allah's forgiveness upon him and his followed, and the salah of the muemineen/believers upon him is du3aa/supplication.
the salah of Allah is not only on the prophet but also on the ones that Allah want to guide from darkness into His light, as we have been told in Quran; " It is He who confers(salah) blessing upon you, and His angels [ask Him to do so] that He may bring you out from darknesses into the light. And ever is He, to the believers, Merciful." (33:43)
not only, but also the messengers (salla Allahou 3alayhi wa sallam) was commanded to send salah upon us and it is considered reassurance for us; "... and invoke salah [Allāh's blessings] upon them. Indeed, your invocations(you salah) are reassurance for them. And Allāh is Hearing and Knowing."(9:103)
and when he(alyhi essalatou wassalaam) was asked how is the way to send salah and salaam upon him he said , say; "Allahoumma Salli 3ala Mu7ammad wa 3ala Aali Mu7ammad kama salayta 3ala Ibrahim wa aali Ibrahim innaka 7amidon majeed. Allahoumma barik 3ala Mu7ammad wa 3ala aali Mu7ammad kama barakta 3ala Ibrahim wa aali Ibrahim innaka 7amdion majeed." = O Allah confers your salaah(showers of nearness of mercy) upon Muhammad and upon the family of Muhammad as you conferred salah upon Ibrahim and the family of Ibrahim, you are Praiseworthy, Honorable. O Allah bless(increase the good) upon Muhammad and upon the family of Muhammad as you blessed upon Ibrahim and the family of Ibrahim, You are Praiseworthy, Honorable.
as he also said, "the most stingy among you is the one that I was mentioned at him and he did not send salaah and salaam upon me."
and he said; "it is from the best of your days the day of Friday/Gom3a,..., so increase from sending salah upon me, it is displayed/presented on me." and someone said,"and how is that when you are dead?" so he asws replied, "Allah forbid the ground from eating the fleshes of the prophets."
as he also said; "there is nobody who will salute me except that Allah will re-bring me back my spirit until I reply him(her) the salaam/salutation."
and he alayhi essalatou wassalaam said; "the most one who deserve me in the Day of Judgment is the most in sending salaah(of Allah) upon me."
and more; "whoever send salaah upon me, Allah will send salaah upon him(her) ten times."
and he said; "Do not let my tomb be a feast, but send salaah upon me. Indeed your salaah will reach me wherever you were."
and he taught us when we do supplication; "when someone do supplication let him start it by praising Allah and compliment words about Him and then send salaah upon me then he ask supplication/duaa as he will/want."
and it is preferred to not only start our du'aa with salah and salam upon the prophet but also to seal it with it, so Allah by His generosity will accept our duaa(if He wills) which is made in between our two salah and salaam upon the prophet(which is not rejected by His mercy).
Salaamou alakum. and salaam upon His servants that He chose.. and our last duaa is elhamdoulellahi rabb-el 3alameen.