I can only give my own motivations for making stuff available as freebies.

Some things I create for a specific use for myself and once they've done their job I might as well let others enjoy them too.
Some figures I do work on to the point that they are functional and do the job they're intended for but then I take them no further (so things like large amounts of morphs or fancy textures people would be expection from a product sold don't exist for them). This often ties in with me experimenting and trying to learn on them.
And finally there are things I just don't think anyone would waste money on - I don't believe they will sell. So I make them free instead