Hardware drag scrolling, combos and snippets, more configuration then you can swing a stick at, layers and all of that. Half of the reason I use QMK boards is due to on-host configuration being so so terrible. At least if I bring my own hardware I _know_ it's going to work how I expect when I plug it in. That's a huge sell if you are jumping between computers all the time and have hardware that fits in your purse.

Also, the form factor and things like the Ploopy Nano are super cool. And because it's open source if you don't like the hardware/software you can easily change it. We use interface devices all day, everyday. Not having an ergonomic interface will catch up with you.