I have a great set of Lock 'n' Lock plastic storage containers that I love and use constantly. I use almost all the sizes regularly, although some more than others, except for this one set. I was never sure why they were included or why anybody would want them. It's a split container, and there's no lid. I could never figure out why one would want a container with no lid. And there were rarely times when I wanted a split container, although they did occasionally arise.
Well, I needed something to stick some change in for my kids' craft sale the other day, and couldn't find anything! I needed separate compartments for the different coins. Then I thought, hey, if I grab two of those split containers, that will give me 4 containers for coins! So I got them both together, but then I needed some way to keep them together and to have a lid on my change box, so I thought, maybe, maybe they'll fit in one of the larger containers. So I grab a larger container and go to place them in it... and both split containers fit in *perfectly*. And that's when it becomes obvious that they were *meant* to, that that's exactly what they're for. Oooops.
(It's possible that originally I knew this, when I first bought the set and read the paperwork. However, if so, I didn't need to use the split containers for such a long time that I forgot their function.)