Sweepstakes casinos run on two virtual currencies instead of cash. Gold Coins are for play only, and Sweeps Coins are the ones that can be redeemed for prizes, including cash, once the site's conditions are met.
I've read through the published redemption terms across top sweepstakes casinos, and this is the plain-English version.
The short version
Gold Coins (GC): no cash value, can be bought or earned, never redeemable.
Sweeps Coins (SC): can't be bought directly, come free with purchases and promotions, and can be redeemed for prizes after playthrough.
Redemption: requires playthrough, a minimum balance and ID verification.
Legality: set state by state, so your location decides whether you can play at all.
Why two currencies...