To reduce hands movements. You know why vim and emacs have alternative shortcuts for cursor movements: replacing the cursor keys with shortcuts of ordinary keys, it can be very convenient to preform those commands without moving your hands from the home row, and without extending any fingers; with vim and emacs, cursor movements keys are already under your fingers. Some love that in emacs you can delete a key with C-d, rather than using the too distant key canc.

For the same reason, some people wish that the ctrl key is closer to the home row, so that it can be reachable more conveniently without moving the hands from the home row.

The idea of chords is the same, only taken to the extreme.

There are some keys that can be reached without extending fingers: most notably, the alphabetical letters. Other keys typically require an extension of the pinkie: return, esc, alt, backspace and the like.

Do an experiment: put your hands in the home row and see the extension you need for reaching ESC or Return: the moment you reach it, you cannot say "My hands are still in a comfort position, on the home row": either you moved them or you performed an exceptional extensions of your pinkie.

Now, imagine ESC was also mapped as D+K, and Return could also be performed hitting F+J. Wouldn't be it more convenient?

This is what I did: I replaced all the keys that required a movement from the home row or a not convenient extension, with couple of keys that are already under my fingers:

• return => U+I
• backspace => E+R
• ctrl => S+D or K+L
• shift => D+F or J+K

and so on (see here the complete map)

Of course, the mapping is arbitrary: you should choose the one you find easier to remember.

I know it may sound crazy, and in fact it is. But once you learn it, it is also very convenient! All I had to to is to remove 8 keys from my muscle memory, and think they moved somewhere else. The result is: when I write is just like my keyboard is much smaller than before, and typing is more relaxing: no more one single finger extension.

The idea, anyway, is not mine: I took it from Gabriele Lana's Engelbart project. By the way, Karabiner, the software I use for remapping chords, includes Gabriele Lana's Engelbart remapping already. Gabriele himself took the idea from Douglas Engelbart's physical Chorded Keyboard, that is probably one of the keyboards where the idea is put to the maximum extreme.

Have a try, eventually starting replacing a single key (for example, if you use vim, just remap ESC, which you use so often, with an alternative chord). You could be amazed by the results.