The module is a charging circuit with a battery attached. Write back is only safe with the battery module. Personally I would not  run the raid without it. It caches data and stores data which needs to be written to disk after a power outage/plug pull/hard reset.   
Have PE 1950 with a UCS-51 SAS controller, but need to purchase a PERC 5/i RAID controller in order to replicate a customer's server. I'm pretty sure WX072 is the model number I need - but there seems to be an optional cache and/or battery pack that comes with it.  I've noticed looking at pictures of it that there appears to be a RAM module fitted - is this the cache?  And is the battery pack used for this, or something else? Please excuse all my dumb questions, just trying to figure out if these are absolutely required, or optional to make it run faster or something?
Question on PERC 5/i controller battery pack/cache