Some do, but it is not required. The Gridcoin blockchain rewards the workforce of each project for free at some base level. If the researchers want to attract even more compute power, they have the option to 'rain' GRC onto their workforce through the Gridcoin wallet.
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So is it similar to how the Steem blockchain pays content creators and curators but Steem users can also directly tip the content creators in order to attract even more quality content?
That is a really, really good analogy! Never thought of it that way, but the parallel is perfect.