Why did He need a pure womb? He came into a sin cursed world. He can handle contact with it.
We need help ~ everything is not clear, and our sector in this part of the universe is rather violent and destructive ~ we barely hang on ~ in the 4 billion years there's been signs of the most primitive life around here, it's only in the last half billion we've been able to find enough stability to work out the code for making brains and eyeballs!
God sends us messengers and moral teachers ~ and as Jesus reported, it's a tough life ~ so it wasn't our imagination. I can easily see a need for God to use a special mechanism so the messenger may arrive intact and ready to do whatever it is a messenger needs to do. This messenger has to live in our world, not just look at it as apparently do the creatures called angels. So, the Immaculate Conception, the Virgin Birth, the Perfect Man ~ the Physical Assumption into Heaven ~ it all sticks together as a great machine that translates the fabric of God's universe to our universe and allows Him to speak to us.
He must think we're pretty special even if we don't think that ourselves.
**Why did He need a pure womb?**
Duh!
Because he was without sin.