This is spot on. There's also the just-so-happens factor, for lack of a better term. A was designed to achieve B but it just so happens that A is actually a very useful way to achieve C.

C was never in the plans of team A, but hey, A achieves it.

If the just-so-happens factor is present then those looking to achieve C get the benefit of all the work and investment done by team A. If this work and investment is substantial then this can make A a great way to achieve C.

Lots of examples. Slack was designed as an internal tool for an indie game studio working on a goofy game called Glitch (screenshot below). Like that's all it was ever supposed to be for. But *as it turns out* Slack was very useful for internal business comms outside of any game context. That's how it goes sometimes.