For me, the JD40 was an exercise in how I could design the smallest possible, rectangular, traditionally staggered, keyboard that included all the alphabet characters, with a standard sized spacebar and a few modifiers. I knew it would be fully programmable, so gamers could program their most used keys and have a very small footprint keyboard to use along with their mouse.
The JD45 was all about making the 40% form factor into a usable keyboard for daily use. The extra column really helps with finger placement, and it also allows for non-QWERTY layouts, such as Dvorak or Colemak.
I build them because they are interesting, to me at least, and not just another TKL or 60% design.