They allow a gamepad to send keyboard/mouse commands by mapping each key, e.g. on an old Playstation pad you could map the X button to right mouse, so you could fire, and left analog stick to mouse movement, R1 for F (use shield bats). Completely customizable.
For even more customization you also use AutoHotKey to remap certain KB keys, assign more than 2 alternative keys etc.
I have it all set up. I don't chain fire. Personally, I see no real gains from doing it. When I played mechwarrior, then it was a valid tactic, but IMHO not in this game. I just like it for the ease of nit having to have an app that involves and extra and honestly unneeded hot key combination.