Lacquer (specifically nitrocellulose lacquer, not the modern catalyzed concoctions) is often used because it is easy to apply, and more importantly, easy to renew. Lacquer does not need to be stripped or sanded to allow another coat of lacquer to adhere to it. Like shellac, each succeeding coat will soften and bond to the old finish. Lacquer can be removed, if desired, with a Q-tip and lacquer thinner. Urethane finishes are extremely hard and almost impossible to remove easily. Application of renewal coats would require roughing up the surface to achieve a mechanical bond (think sandpaper on your color cased frame edges), whereas lacquer will form a chemical bond as long as the old coating is clean. It's a good idea to clean up the old finish with lacquer thinner so that you don't build up too much film thickness. The clear coat should be microscopically thin. I prefer Tru-Oil for its ease of use.
