It is a box that I put pieces of molding and other small parts I have made for various pieces of furniture. I have enough examples in here by now that when I am considering a cove or what ever for a new piece I can get an example of various sizes from the box. Having a piece of molding in hand, or a pull, is better than a drawing to determine if it will look right on the new piece. If it is molding I write the cutter I used to make it on the piece.
Especially useful are profiles I have used for case tops of various thicknesses and styles.
I've done essentially the same thing years ago. I've got a box of profiles I've cut and joinery sections like cope and stick and such. One of the useful things these parts do is help with setting up the router table next time.
I would add, if you have a dovetail jig then save samples of all the tail and pin stock and write down the number of the dovetail bit used to cut them. It will save you countless hours of setup time since it can easily reproduce the proper depth setting for each sort.
I do a similar thing. I keep samples of all the picture frame profiles that I've made so I can show possible designs when someone wants a picture frame.
