If you don't use earned comps in a six month period, they are taken away (I found this out the hard way, when I first started playing there). If your comp balance exceeds 3000 pts, the excess is also forfeited. So if a host comps you directly for dinners (say), and you don't use your points--they could disappear.
One can redeem points for Borgata certificates (min $50) for use in Vegas.