First of all, if you want to do crafting, you need ALL the crafts up.

For example, failing to level CUL will mean you miss out on Steady Hands 2 and Hasty Touch which are two of THE most important skills on the toolbar. Not leveling Alchemist means you miss out on Comfort Zone which is also a big part of the routine.

Secondly, you can repair your own gear which is always useful, and there are a few fringe recipes around that can be money-makers, and sometimes you just need to make stuff for your own use. It isn't about what you can make for yourself, though there typically are master recipes that are as good as dungeon drops (I know there are 320 craftable weapons and during the last patch of Heavensward, there were craftable armors that were comparable to Baelsar's Wall drops).

I know that certain glamour gear, such as the swimsuits are going for 200k to 1.2m on Lamia, I just checked one of the swimsuit bras and it was 1.2M and there were only 4 up on auction. Seriously, if you could get the recipes and mats for those, you'd be making a ton of money.

You can also meld your own materia if you had your crafts leveled up, which saves you the fees from getting the NPC (or someone else) to do it.

And, well, leveling the crafts can be fun if you're into that kind of thing. I couldn't imagine what I'd ever do with my time in-game if I didn't level any of the crafts or gatherer classes. I'd be out of stuff to do.