Hi Tony,
John P has this right. I believe the 6-1/2 or 6+ fret became common in the 70s and is almost standard now, to the point that a dulcimer that does not have that extra fret will usually be labeled explicitly asa "true diatonic."
The 1-1/2 or 1+ fret is the next most common "extra" fret. It is useful for blues, as John P states, because it gives you the lowered third, the most important of the so-called "blue" notes. But it has other uses as well, most notably the ability to play the 7th chord of the I and IV and also a wider variety of minor chords.
Two of Bing Futch's recent Dulcimerica videos examine the 1-1/2 fret in depth, so if you want to see what can be done with that extra fret, give those videos a look.
I've seen some folks who have added a 4-1/2 fret, and Ron Ewing convinced me to put not only a 1-1/2 but also a 1/2 (0+?)fret on my dulcimette, the logic being the greater number of chords that could be played. I'll be honest, though, although I use the 1-1/2 fret often, I have not yet begun to explore the .5 fret (or however you designate it).
Traditionalists will argue with some pursuasiveness that once you begin adding extra frets, the dulcimer ceases to be a dulcimer and becomes something closer to a guitar or mandolin.
There is a group here at FOTMD entitled "I have extra frets," which sounds like the opening line of a 12-step group: "Hello, my name is Dulci and I have extra frets.
I knew I had a serious problem when after getting a 1-1/2 fret one day I was banging on my luthier's door at 4:00
AMbegging
for more."