Running full nodes is no longer really profitable due to cost and technical complexities associated in setting it up. And this causes a decrease in the number of full nodes so as to keep incentivizing operators running full node, another way was deviced which is master nodes.
Master nodes are full nodes that allow operators to earn by performing the function of keeping the blockchain in shape. Although, masternodes helps increase the efficiency of the blockchain but with few full nodes, we will see network congestion and slower confirmation time. The first cryptocurrency to implement masternode system was Dash.