Buttons can do lots of things - expand disclosures, pop dialogs, play videos, toggle themes, slide views, download files, and so on - and disabling the button may be a least-bad method of signaling a feature is not currently available but may be later for reasons that you have made clear to the user.
The place where buttons should not be disabled is within the context of submitting a form. Or, more specifically, within the context of gathering information from a user that may need to go through validation, review, corrections, and so on.
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