For a centralized application to sustain for a long time, the owner of the app needs to make a profit in order to keep it running. DApps don't have an owner, but still, like any other centralized app, the nodes of a DApp need hardware and network resources to keep it running. So the nodes of a DApp need something useful in return to keep the DApp running. That's where internal currency comes into play. Most DApps have a built-in internal currency, or we can say that most successful DApps have a built-in internal currency.
The consensus protocol is what decides how much currency a node receives. Depending on the consensus protocol, only certain kinds of nodes earn currency. We can also say that the nodes that contribute to keeping the DApp secure and running are the ones that earn currency. Nodes that only read data are not rewarded with anything. For example, in Bitcoin, only miners earn Bitcoins for successfully mining blocks.
The biggest question is since this is a digital currency, why would someone value it? Well, according to economics, anything that has demand and whose supply is insufficient will have value.
Making users pay to use the DApp using the internal currency solves the demand problem. As more and more users use the DApp, the demand also increases and, therefore, the value of the internal currency increases as well.
Setting a fixed amount of currency that can be produced makes the currency scarce, giving it a higher value.
The currency is supplied over time instead of supplying all the currency at a go. This is done so that new nodes that enter the network to keep it secure and running also earn the currency.
