How is a finish sander different than an orbital sander?
Anything that sands across the grain on wood puts scratches in the growth rings that can be hard to remove. 

Orbital sanders are good for removing homogenous materials like paint where you don't want a pattern to the marks left by the sand paper. As you use finer paper, the random action leaves finer scratches till the surface is smooth.

Finish sanding on wood is a bit different, you want a straight line sander so the abrasive cuts parallel to the wood grain so you can get a smoother surface.

Also, Finish Sanders are available with Random Orbital Action. If you've ever been in a car paint shop, you know it as a DA or Dual Action Sander. The pad is loosely coupled to the orbital mechanism so it doesn't produce spiral cuts in the finish but has a random combination of straight and orbital motion.