When would I want to use the assign host later selection?
If you want to speed the process for creating volumes, you can skip the host assignment step so that newly created volumes are initialized offline.
Newly created volumes must be initialized. The system can initialize them using one of two modes — either an Immediate Available Format (IAF) background initialization process or an offline process.
When you map a volume to a host, it forces any initializing volumes in that group to transition to background initialization. This background initialization process allows for concurrent host I/O, which can sometimes be time-consuming.
When none of the volumes in a volume group are mapped, offline initialization is performed. The offline process is much faster than the background process.