Question: why did aizen want orihime?
Answer: Aizen has many plans to manipulate and use Ichigo, and so kidnapping Orihime was a good way to motivate him to enter Las Noches. He wanted to lure the soul reapers to Las Notches and lower their defenses as he destroyed Karakura. He wanted to use her as a pawn to lower the military power of the soul reapers.