This was in accordance with Surah Ahzab (Quran 33:56). The Surah was revealed in the context that Hypocirites and People of the book were planning different schemes to malign the household of the Prophet to ultimately put a smudge on the Prophet’s (sws) character and thus damper the Islamic movement that was spreading in medina and outwards. There were many directives given in that Surah to counter that situation like the wives of the Prophet were given special guidelines to counter the scheming of hyporites and other directives as well. It was in this context, the Muslims were asked that in the face of all the scheming going on to scandalize the character of the Prophet, Muslims should send blessings on the Prophet as a response. There were no words prescribed in this regard, one can send belssing in any noble way. But when Prophet (sws) was asked how to send blessings on him, he recommended this darood and Muslims carried forward this tradition. Therefore, in that context, sending of blessings on the Prophet and his household was the appropriate thing to do and we Muslims still do that out of our love and respect for the Prophet (Sws).