As a Quick note, I am a Container Junkie silly as it may sound. I often Buy containers just for the sake of having them around and most of the time I will think of a Need and instantly go through what containers I own and 90% of the time I will have the perfect container that just works like it was made to be.
I'm a container junkie, and especially a pouch/pack nerd! Since I'm not rich, I really fetishize Maxpedition stuff. If I had more money I'd probably be drooling over Kifaru and Mystery Ranch gear but Maxpedition is about the top of my budget for EDC and misc stuff.
Sometimes the container suggests itself. For instance, one of my most carried PSKs rides in my Maxpedition 12 x 5 water bottle carrier. I always carry water anyways, so I've added as many things as I can to the carrier. The result adds little weight compared to a SS bottle with 40 oz of water, yet although it's not a lot of gear it probably would be enough to tip the scales in my favor if an outing turned against me.
An Altoids tin is the classic case of chosing the container first- and of course, I'm not saying there's anything wrong with that. So long as one has decided that's the maximum space you can allot and chose the best gear you can given that space limitation, the Altoids kit can be useful.
My own classic case of choosing the container first, aside for the previously mentioned bottle kit, is the lumbar pack. As a day pack I find them pretty comfortable vs a regular backpack, especially when it's hot. I will add PSK items that work with the stuff I'm already carrying (eg water, snacks, etc). Of course, this has to be guided by a disciplined analysis of what you might need vs what would be cool to have.
Again, it's not a revolutionary idea or anything. I'm just trying to move away from "how much crap can I fit in this thing" to "what do I really
need to carry?"