why do we need radio interfaces in AP. what is the purpose of creating sub interfaces.
Sub interfaces are used so that you can seperate different "vlans" on the Radio/Ethernet. Think of the Access Point like a Router (but it can't route). The access point needs subinterfaces and bridge-groups in order to bridge the traffic between the radio and the ethernet much like you'd need subinterfaces and bridge-groups to bridge the traffic between different layer3 interfaces on a router.
If you don't want to manage/configure sub-interfaces on the radio, I'd suggest you look into the Controller-based deployments.
Bottom line however, is that if you want multiple vlans on your wireless Access Point, you need subinterfaces for each vlan on the Radio and Ethernet, as well as bridge-groups to bridge the traffic together.