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Creators making their content available for free and saying you can give them money if you want to, as opposed to just selling their content, is a business model that long predates vtubers. People creating videos, comics, games and other things online have been doing this for many years. Viewers who enjoy the content give the creator money so they can continue to create the content, as they'll likely have to stop if they can't make a living off it.
Often you can earn extra perks for paying, such as early access to content or a couple seconds of the creator's attention, but those are usually fairly minor.
