It allows the raid card to remember what is in its buffers ( that hasnt been sync'd to disk )
Its very important for people who need high data integrity.. Or to save your DB from certain types of corruption..
(Basically whats on disk, is on disk - so thats safe.. The problem is when the OS thinks its on disk but its actually not and in a RAID card buffer)
When the server starts up again, obviously those buffers get flushed to the disks.. So you have a point in time correlation with your disks and OS..
( otherwise you will just loose information - like a few database records, which you will never know. )
A UPS help sure..
but its not safe enough.. ever decent RAID card should have a BBU (Battery Backed Unit)