1- Allah wanted to try the Prophet (s) and elevate his status. It’s not easy having to deal with such wives.
2- To teach others and be a role mode in dealing with difficult spouses. Despite their conspiracies and troubles, the Prophet (s) always maintained his decency and Akhlaaq with them. The same applies to Prophet Nouh who was married to an evil wife according to the Qur’an.
3- To test this Ummah. To see if they follow God or Aisha. Imam Ali (a) said this once. He said “Allah tested you with your mother (referring to Aisha) to see if you follow Him (by siding with the truth and being with Imam Ali) or follow her (which many people did and failed their test). So it was a major trial for Muslims.
We dispute that she (Aisha) was 9. The numbers don’t add up. It seems that she was around 16-18