It's also possible that successfully throwing an attacker could have a greater demoralizing efect on them--if you strike someone and don't produce a knockdown or knockout, they A) are still standing and thus still mobile, and B) know how hard you can hit and aren't afraid anymore.
Putting them feet, body, hands off the ground and into something harder than anything you were born with A) gives you time to run/follow up as appropriate and B) sends a somewhat stronger psychological message that they just got taken *down*.
Now with all that being said, there's aleways that rare guy who is either immune enough to pain or wants to hurt you so bad they just don't care......not over till it's *over*.