It sets the value which is most important. Then, it's a matter of glazes, tweaking, scumbling, and polishing. Did I mention that with egg tempera you can get a nice sheen and temper the surface by hand polishing with a soft cloth. Plus, no risk of lint or hairs getting trapped like a (fly in amber) for all eternity. (Often happens when you varnish)
The values are lighter than the end result will be because each glaze layer will darken the value. Any areas that become too dark can be lightened by scumbling a lighter tone over the area. It's all about making adjustments until I am satisfied. 99% of a painting is getting the values right. The rest is emotion, draftsmanship, and surface quality.