Why did the Messenger of Allah (peace and blessings be upon him) marry "Maryam al-Qibtiyyah" when he already had wives at that time?

The Shaykh : 10: Why did the Messenger of Allah (peace and blessings be upon him) marry "Maryam al-Qibtiyyah" when he already had wives at that time? And is there a Shari'ah reason or explanation for the fact that the Messenger of Allah (peace and blessings be upon him) did not free her and then marry her?
This question actually contains two questions. The first question is: Why did the Messenger of Allah (peace be upon him) marry Maryam al-Qibtiyyah when he already had wives at that time?
I say with complete frankness: I do not know why he married her. I only know for certain that he did what Allah permitted him to do, and Allah permitted him to marry as many women as he wished. As for why he married that specific woman, I do not know the secret behind it. But I also know for certain that he married her for her benefit, not to harm her, and to protect her chastity, not to lead her into sin. There are other benefits that cannot be fully comprehended. Some Islamic writers have written acceptable works explaining the rationale or philosophy behind the Messenger of Allah's (peace be upon him) marriage to his many wives. They explain and justify why he married each one. For example, he married Umm Salamah because her husband had died, leaving her with young children, so he married her to be their guardian, to raise them, educate them, and provide for them, among other things. He married the Lady A'ishah because she was the daughter of his companion in the cave. He married Hafzah, and so on.
They find valid reasons for each wife for which the Messenger of Allah (peace be upon him) married her. However, the matter is broader than that; a person cannot fully comprehend the secrets and the wisdoms for which the Messenger of Allah (peace be upon him) married the women he did.