Bags kind of suck, but if you're the sort that gets meticulous about inventory arrangement or collecting, they can be handy. For roleplaying purposes, I used to have an Action Historian character that kept a survival kit (lighter, rope, edibles, rad chems, first aid kit, nuka, Cat's Paw) in one and every holotape in the wasteland in the other. You right click and use the hand icon to open or close them, and you can drag items over the top of their inventory icon to place them inside, IIRC.
Once or twice, you'll find one placed in the environment that already has stuff inside it, too. Make sure to check the one you find in the Junktown casino before selling it off.
The point of Vault 15 can be easy to miss, as the relevant info flashes by pretty fast in your text window, usually during a running rat fight.