I've seen 6-strings (3 courses of 2 strings) for the sake of volume, in the same way that most dulcimers have a double melody string. My other thought would be 6 equidistant maybe for fingerpicking (at least, people use 4 equidistant for that).

I don't know--a friend of mine recently sent me a chromatic lap dulcimer to test. I said it was fine but when I start seeing that many more frets and strings, I began to think of just taking up the mandolin.