Hi Jeremy,
I generally think it is easy to stray away from reality when one inserts the word "everyone" into a topic. Unless you're speaking of death. Taxes we know no longer applies to everyone, so that's changed.
But really, the same could be said about Facebook Marketplace and Craigslist. They are platforms that duplicate the same service. Why? Why not?
And back to your question... Posting on the SFAF website is local, generally, and may save yo the hassle of shipping something out after a sale if a local member or buyer is interested. And there's no rule that you can't place multiple ads on multiple websites. I see this done often by many listers.
Cheers,
David