The usual answer to that is "discoverability". You can either post your content to someone else's website and have it read/viewed/watched, or post it where virtually no one will see it so you might as well not post it.
Yes but discoverability to what end? In one case the reader then thinks and says "I learned this on Stack Exchange" rather than a possibility of "I learned this from Mark".
In one case an organization that is much complained about becomes even more of a necessity and so has less incentive to fix itself. In another the reverse happens.
In hyperbole: in one case you may as well not post it, in the other case you may as well have not written it.
