When using dynamic allocation, you request I/O resources by coding the DYNALLOC macro and filling in the fields of the SVC 99 parameter list. The system allocates and deallocates those I/O resources while your program is running. Dynamic allocation also allows you to request information about your allocation environment, and to deallocate or modify characteristics of your allocation environment that were acquired either dynamically or through JCL.
Dynamic allocation allows you to tailor your device allocations based on input to your program. You can design your program to dynamically allocate only those devices that are necessary in a particular programming path, rather than allocating all possible device requirements before your program runs.
Dynamic allocation also allows you to use common resources more efficiently. When there is high contention for a resource, dynamic allocation allows you to acquire an I/O resource just before you need it and to release it just after you need it, so that your program holds the resource for a shorter length of time.