Why did the holy prophet marry Hazrat Maymunah Bint Al-Haritha?

Maimuna or Barra as she was then called, yearned to marry the Prophet. She went to her sister, Umm al Fadl to talk to her about that and she, in turn, spoke to her husband, al-Abbas. Al-Abbas immediately went to the Messenger, peace and blessings of Allah be upon him, with Maimuna's offer of marriage to him and her proposal was accepted.

When the good news reached her, she was on a camel, and she immediately got off the camel and said, "The camel and what is on it is for the Messenger of Allah, peace and blessings of Allah be upon him."

They were married in the month of Shawwal in 7 AH just after the Muslims of Madina were permitted to visit Makkah under the terms of the treaty of Hudaybiyya to perform Umra.

Allah Almighty sent the following ayah about this:

Any believing woman who dedicates herself to the Prophet if the Prophet wishes to wed her, that is only for thee and not for the believers. (Al-Ahzab : 50)