They charge based on API request numbers.
Why they exist? Is it useful? Time saver?
How many time saving if use it?
People have mentioned some of the various advantages. But I think a good real world example might be people who subscribe to the Bloomberg terminal. It allows you fast access to market numbers and news. It's good enough that they manage to charge I think around $20k per year for it. And they have like 300k subscribers. It's quite amazing really. It's not hard to see why people are trying to replicate this premium model type offering in the Bitcoin world.