The prophet had many rules only meant for him. Like he would continue fasting without stopping or stay up all night making Salah or marrying more than 4 and all these had to do with his role as a prophet. Other prophets were like him too, Sulaiman had many wives etc.

 

One of the main reasons for his multiple marriages were teaching his wives matters of Fiqh inside the family so they can narrate it to Muslims.

 

So we do not explain the reason for his marriages in political matters but in the justice he was able to treat all of them, the education he gave them, the honor they got being family of the prophet, the social model that all his wives were either divorced or widowed except for `A’shah etc.

 

Almighty Allah knows best.