Mike, a thin coating of a compatible varnish, such as artist's acrylic gloss varnish, protects the printed image on the carrier film, reducing the risk of it breaking up when soaked in the water, and, more importantly, when it is moved, to remove it from the backing paper.
Depending on the type of printing process employed in their production, most commercial decals have a finishing coat of some type of varnish anyway, but, after time, this can decay, or harden, which can then cause the decal to dissolve, break, crack, etc etc, when used.
A 'fresh' coat of varnish, as described, reduces the risk of this happening, on older decals, although does not guarantee to work every time.
The 'paper' on the decal is only a carrier vehicle, with the decal 'held' onto this backing by a water-soluble adhesive, which dissolves when the entire decal is immersed, allowing the printed image, on its clear carrier film, sealed with the manufacturers's varnish, to release. A further coat of 'new' varnish should not have any detrimental affect, although it might take a little longer to release from the backing when soaked.
You are correct in thinking that 'extra' varnish may affect the 'draw down' and conformance of the decal, but this is fairly minimal and, especially with a decal softening solution, such as 'Micro Sol', there is no reason why a decal so treated shouldn't perform 'as normal', albeit perhaps a little slower than if 'new'.
Back in the 'good old days', before the advent of after-market accessories, and especially before home computers and their associated printers, modellers, including a then young 'me', often made their own decals, by varnishing ordinary gummed labels, hand-painting the required design, and then varnishing this which, once fully hardened, was used in the same way as a commercial decal.