I probably should have elaborated on the varnish a bit more in my Rivi thread so let me do so now.
Not all woods have it but, certain ones have a complex grain that can be multidimensional when finished correctly. It's a term called Chatoyance (flame mahogany, burl walnut and figured maples are good examples). It's a French word meaning "cat's eye" and pretty much sums up the effect. It will give you a shimmer or almost 3D effect when you move the wood around in natural light. Keep in mind that not all woods can produce this effect.
You may be able to see some of it with just a general clear coat finish but in order to really make the grain pop, you have to use a penetrating oil to make it dance. Linseed oil would be the go-to but I opted for Varnish because the effect was greater than the samples I made with the boiled linseed oil and I wasn't comfortable using straight oil under the clear (varnish is a mixture of oils and resins). I used Old Master Tung Oil Varnish.
To be clear; the varnish is not used as a finish but a penetrating oil. You don't want to build a traditional varnish film as you would if you were finishing the wood with varnish. Instead, you simply want to wet the wood to get the oil in the fibers and then wipe the finish off after sitting for a few minutes. You also don't want to let it sit and saturate the wood since you are using veneer. Unless you are using paper-backed veneer, using too much oil can cause it to seep through and plasticize your adhesive film....and potentially delaminate the veneer.
Instead of just rubbing the varnishing with a rag, I did it with with fine grit paper to create a swarf (oil mixed with the sanded wood dust) to create a bit of a natural filler for the pores instead of using a pore filler. Again, I did so because pore fillers (not the clear ones) use pigments which can bleed into the wood fibers and muddle the grain.
The key to this method when using a clear top-coat is to wipe the surface of excess oil and do not let it build. I kept wiping over a few hour period since oil will continue to bleed out slightly over time. The other import point I made in that thread is that the veneer was allowed to cure for a week. That allowed sufficient time for the oil to "dry" before top-coating.
Again; the varnish was just part of the finish process for a final effect. Hope that helps.