Apparently God wanted a pure receptacle for the birth of His Son. Of course it wasn't necessary or essential. But, He decided on creating another human without sin. Mary was sinless at conception and remained so always. She didn't HAVE to but she did.
Remember, Adam and Eve were created without sin. They CHOSE sin.
God also decided to send a SON, not a daughter. There were many strong, holy women in Jewish tradition. Jesus MIGHT have chosen 12 WOMEN as His Apostles, or half and half. He might have made Judas Iscariot a woman. But God chose whom He chose.
You can ask Him why He made those choices when you meet Him.
I plan on asking Him what the deal is with the structure of the knee...why so complicated, Lord?
Im glad you agree that the immaculate birth wasn't necessary. The theological mathematics that deem it necessary are presumptuous and silly.
What evidence is there that God chose to make Mary sinless?
What would chairs look like if our knees bent the other way?