It's obviously not "used" for anything, at present, being cut off; At least, this particular piece is not in use. What you need to determine is whether its still connected at the other end, so you need to follow it back to where it comes from.
While you could certainly just carefully wrap the end in electrical tape for safer handling, you'll want it disconnected (not merely turned off at the breaker, if it's still connected to a breaker) unless you happen to have some use for it (nice, heavy gauge cable by the look of it.) Might have been for an electric dryer at some prior time, for instance. If you have no use for it, remove it and sell it. Copper wiring is not cheap these days, especially in large sizes.