Without striping, the system may write all data to one LUN until it is full and then move sequentially to the next LUN. I'm not certain it will do that, but it certainly data is probably not well distributed if not using striping. So for large writes, you may be stressing one LUN at a time.
Striping will put data in each of the LUNs so that each LUN is utilized equally at all times. The big benefit is that striping will always give you 4 active threads in your example instead of potentially a single thread. On the FAS8080EX, multi-threaded performance is always better than single thread.