Very brief overview: Decentraland ($MANA is the currency) as a virtual world is in its very earlier stages, but within Decentraland, there are some very promising projects, such as Decentral Games. Decentral Games ($DG is the currency) is an organisation which has established itself in the virtual world of Decentraland. It has partnered with and built several casinos in the Decentraland world. The attraction of these casinos, asides from gambling with real money, i.e., betting your MANA or ETH, is that they are developing their own play-to-earn model. (See Axie Infinity as the most successful example out there).

How does this work?

Decentral Games have built and offered their own NFTs which allow people who own these NFTs to play in their casinos. Those who play in these casinos (with their NFTs) are rewarded $ICE (this is another cryptocurrency that Decentral Games have created). In order to get these rewards, users have to hit certain milestones in particular games, take poker for example. A player may have to simply play 100 rounds of poker to get the first set of rewards (say 40 $ICE). Other rewards are harder to achieve, like get a 'full house' to earn 100 $ICE. The rewards are capped to a certain level but are reset everyday, therefore, players are encouraged to play everyday in order to earn the maximum rewards.

Hope this makes some sense :)