Witnesses get a producer reward for resolving a block on the blockchain - this only happens if their Witness server is active and processing transactions at the time it is callled to resolve a block. The frequency a Witness is called is based on the ranking - which is influenced as you now know by the Steem Power held by the voters. The higher your position the more frequent the producer reward - right now I get called for a block every 5 hours and receive 1 Steem Power in return. The top 20 are getting called for blocks every few minutes. The rewards need to contribute to server costs - which are expensive. And the top 20 run nodes as well which are even more expensive and have to be on very fast gigabit up and down internet connections to - most Nodes are on 512gb Ram and a few terabyte of SSD storage. They have to be as they form the main backbone for keeping the transactions on the Steem blockchain as fast as they are- in fact Steem is one of fastest crypto currencies for transaction speed.