From my own experience, I'd say that for acrylics, a varnish layer will the give the painting surface a more even sheen. Look at an acrylic painting in different angles of light and you will probably notice patches of the canvas that have a different surface sheen then others, some may appear flat, while other areas shiny, etc.. A varnish layer will help smooth that out and give the entire canvas a nice even surface consistency, which makes the painting much easier to see in a variety of different lighting environment, in my opinion.
As for archival protection.... I'm no expert on this. I know most varnish has UV protection, which should protect against fading and color shift ("lightfastness"). I've been thinking about doing an experiement on my own to see this protection in action. The idea was to paint two large colored squares on canvas of the same color, varnish one square but not the other, then leave the canvas in my window in the direct sunlight and see what happens.
Dirt, dust, smoke (say 10-15 years worth) can also stain a canvas pretty heavily, and without a varnish coat, I'd imagine it would be tough to clean an acrylic painting and get all the gunk off without compromising the paint. Varnish provides a layer that makes it safe to do a more heavy cleaning of the painting surface (providing the cleaner used is not one that will penetrate the varnish). I've also heard that acrylic paint is self-sealing, but I'd imagine that depends on the amount of paint used, I'm not sure I'd trust a thin or transparant area of the painting where the canvas shows through quite a bit to a heavy soap&water scrubbing without knowing a solid layer of varnish was there for protection.
Again, I'm not an expert, so if anyone wants to offer clarification, please do