The coco mat can help with a few things, one, helps keep a little moisture on the bottom of the table because some of the coco cubes can dry out fairly quickly, also it does give the roots something to grow down into which unlike the hydro rocks, will protect the roots from light and help to stop roots from clogging up any drain pipes. One thing i've notice from another grower is that with the 2 sizes of thickness which the mats usually come in, the thin one is as good as the thicker one and cheeper.
Adieu