By thoroughly coating all sides, water is denied entry that would swell the wood and ruin the art. Of course, it is always a good idea to consult a person knowledgeable about these materials before plunging ahead on a project so that you maximize the likelihood of a satisfactory outcome. Soluvar, or it's generic equivalent, is the only varnish that will not ultimately destroy an acrylic mural. For this reason it is widely used in conservation labs as a final or isolating varnish on acrylic and aged oil paintings. Soluvar is far less porous than acrylic varnish and so can create problems when used on a mural on a damp wall, since the wall moisture mixed with sunlight and temperature fluctuations will tend to push the paint off the wall.
Water vapor is restricted by the varnish from passing through the relatively porous paint.
The reason I discourage the use of acrylic emulsion varnish on acrylic murals is that the porous acrylic varnish absorbs smog, smoke, dirt, algae, etc., permanently. In addition, the clear acrylic film causes ultra-violet rays to oscillate within it's thickness, causing it to break down and powder very quickly.