Why? Becouse guitars with a fret fingerboard cannot obtain glissandos, quartertone differences in the soundpitch and a clear, profound tone of low (bass) sounds.
In fretless guitars, on the other hand, it's impossible to obtain a clear tone and strong dynamics of higher (violin) sounds.
The number of fretless strings can differ (e.g. 2-not necessarily 3 as in the adjoined picture). Obviously the solution can be employed in acoustic and electric guitars as well.
Latar, the guitar of my construction offer new possbilities, new sound and articulation. It also damands a special way of playing.