The Optimal setting has the machine make more passes over the carving and is a little "gentler" on each pass. It reduces chip-out on thinner and/or delicate areas of a carving and the result is fewer "fuzzies" to deal with when prepping for finishing. In other words, overall quality is improved.
The Best setting usually takes approximately half as long as the Optimal setting, and I use it whenever I can. On finely detailed carvings, or carvings with raster lettering that I fear could be prone to chip-out, I'll go with Optimal. (I never use Draft or Normal settings.)