When purpose is just use What are your thoughts on "structure equals function" in relation to this? On a cellular level, there is not much variance in use for a particular piece unless it is, for example, a duplicate. Something's function and shape influence and match each other, a process that is seen in natural selection. But larger, more complex, and aware things generally have more leeway into what something's usefulness can entail.
We can attribute usefulness to something, that is, a door can be useful for keeping things out or it can be used as a life raft in a flood. Things can also be useful insomuch as we value them. Keepsakes and sentimental trinkets are a good example, as their "usefulness" is in the value we place on them for physically embodying memories and emotions. The door example is more practical, while the trinket one is more arbitrary; however, both are ways something can be considered useful.
The context something is in can also determine usefulness. Fins are useful for moving through the water, but hardly for moving on land. I think it would be possible to conceive a situation where fins could be useful on land, but it probably would not be for movement. This is another idea seeable in natural selection. Things are only more useful, that is, more adapted if they pair well with the environment they are in. I think it was in The God Delusion that Dawkins talked about bird wings and their usefulness. He was saying that having 50% of a wing could still be useful in that, though it couldn't be good for flying, if that bird was in a state of free fall the air resistance it would cause would help to break the fall, even if by just a little bit, which could be the difference between surviving the fall or dying.
I think it is important for us to not count things out based on our current understanding of them. Everything changes, too, so someone that can barely lift an ax today could in theory be the world's strongest lumberjack later in life. I think our potential to change is an inseparable part of what we are and is much broader than we often realize.
Sorry if this is not what you were trying to get at, though.