Gambling sites have different promotions to attract players to use and play on their sites. Most of them give out free coins when you register, giveaways, challenges, competitions and a lot more. I think other gamblers are also looking for site that are generous when it comes to this kind of promotion.
Indeed, gambling site without promotion will not be see and such gambling platforms will not attract good gamblers to gamble on their platform. Promotion is one of the major source of promoting any platform to the public, without the said promotion, now one will care whether such platform exist. For example, Windice.io are currently promoting their casino dice platform through signature, giveaway threads, rains, faucet and those with forum giveaway like Windice have done recently on their forum contest.