This is an often asked question. A satin lacquer is used and for a couple of reasons.
First, it is sprayed with a hot catalyst so the drying time is quick and this is important when working in a dusty space. It reduces the chance of wood particles laying on the surface.
Secondly, it is a clear product that greatly enhances the grain and coloration in the mantel.
Thirdly, it makes me look like I know exactly what I am doing because it is "self leveling" and is perfect everytime. And if it gets nicked or scratched when installing on your end, it is easily repaired in that another touch up coat will melt into the previous coats. Very little sanding, if any, is needed.
The only downside to lacquer is that it is not as durable as some of the other finishes but not much of a problem when one considers the low traffic on a mantel.