So I used crafting catalyst, which I originally wasn't gonna use until I was done killing 15 types of monsters, but because of the fact I got protean hides from the treasure trails, I did it before.

And I got leather case, but what's it used for? Apparently I need another item which I have no idea about.

You can combine it with a metal setting and a gem focus to make a summoning focus. The upshot is that each summoning focus yields 10 bonus xp in Crafting, and each focus gives 20% extra summoning xp when making pouches at an obelisk (one focus consumed per pouch made).

 

During the Treasure Hunter promotion you could swap parts with an NPC in the Crafting Guild. I'm not sure if you can still do that now (probably not) but you are still free to buy and sell them between other players or on the Grand Exchange.