I use it on the fourth gen and earlier games for extra TMs. There's a code for R/S/E that duplicates any item you use so you never run out, and that's the easiest way to do it. For DPPt, I just add extra TMs to my bag as I need them. I do that because I like to use lots of different pokemon through the course of a game, and there's no way that I can pick just one to get a TM move.
Since TMs are reusable in fifth gen, I don't need to do that any more so I stopped. Then I ran across the show IVs/EVs codes, which are great (I used to use an IV calculator to try to figure out IVs), so I've started using it again for that.