These are the reasons that I license my webcomic, JoeRunner and co., under CC BY-SA 4.0.
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Allowing people to redistribute my work unrestricted means that more people see it, and it has a smaller chance of becoming lost media.
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Using a CC license raises knowledge about CC, possibly inspiring more CC content.
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Using CC BY-SA might inspire people to create derivative works also licensed under CC BY-SA, creating more CC content. Internet users like to use copyrighted content under fair use anyway.