Just wondering if anyone actually uses them for something.
Every freaking day.
Almost everyone touched on most of the reasons I do, but I find that if I'm even the slightest overleveled, then the quest rewards can't keep up with me. Neither can the commendation awards.
Example: I'm playing some lowbie characters. I left Coruscant at Level 19. The highest level gear I could buy with comms was Level 15.
How does that help me?
Not to mention that unless you're playing a tank class, it's hard to get the item mods geared for you using comms.
I took Cybertech to start with having no idea what I was doing (I thought it made IC sense for my Cyborg Smuggler), and in hindsight, it was the best 'accident' I could've made. I sell low-level Prototype armoring on the GTN for credits, and use it to keep all my leveling toons geared when the rewards/comms don't cut it. I use Artifice for the Enhancements, Hilts, and Crystals, which are rarer than hen's teeth as a drop in day-to-day play (I had a crystal drop for my Trooper back when she was Level 34, and I realized I was running around with a Level 15 crystal in my gun). I didn't make as much use with Biochem with my Sage while leveling except to have the reusable stim for her, but I'm now using it to make stat-specific Implants for my characters.
And then there's the ability to craft armor /you/ want to wear using Synthweaving or Armormech. There is a lot of really ugly armor in the game, and the ability to make exactly what I want and not have to depend on the whims of people overcharging for it on the GTN? Priceless.
That doesn't even get into the ability to craft endgame stuff, which you can't even do unless you level your crafting skill up to 400.