Similar to PPSSPP, Combo Key feature is basically mapping multiple buttons to a single hotkey. Some games require for multiple button inputs and pressing those buttons feels like you're playing Twister with your fingers. This would be really easy and convenient to those without a controller.
Spoiler!Offtopic: Ive been using Drastic for a very long time now and this is my first time posting here. I just want to say thank you to the devs for the hardwork and effort in making this the best DS emulator for Android.