For ages, the theory was that when handling miniatures, you wanted something between your fingers and the paint, because your fingers will take paint off.
However, modern paints created for gaming miniatures don't really NEED that extra protective coating. Dropping a miniature will have the same effect whether or not there is a layer of varnish on it or not.
Furthermore, if you're using Reaper's Bones miniatures, you don't want the varnish, because varnish isn't flexible. The Bones plastic is, and when you flex the material the varnish will crack. It will take the paint with it. Acrylic paint is surprisingly flexible (there's a thread somewhere over on the Reaper forums about a painter flexing a piece around back on itself and the paint doesn't crack).
As for any other "preservation" of the paints, no, varnish really doesn't do that. You're not putting a thick enough coat onto the miniature. It's really only there to prevent gentle dings and scratches that come with handling miniatures. But Reaper, Privateer Press, and I'd imagine GW paint don't damage that easily.
Edited to add: Imagine putting a layer of clear-coat on your car. It adds a "layer" to the paint, but someone can key your car just as easily as they could before, and it won't do anything if you crash into a telephone pole. There will be some color preservation properties, but you don't leave your miniatures outside for hours at a time.