Let's say you have a bucket, and you're in a large, cubical room. In front of you are two small mounds, one containing dirt, the other containing sand. You have to carry both mounds to the other end of the room without mixing them.
Which means that the empty bucket is simultaneously more useful than a bucket of sand or a bucket of dirt, and less useful.
Think about it.
When the bucket is empty, it can be used to carry either the sand or the dirt, so it's more useful than a bucket with one or the other. However, just an empty bucket will bring you no closer to your goal of carrying the sand and dirt across the room, so it's also less useful than a filled bucket.
...
I'm sure there are like, a million flaws with that, so please point any you can think of out to me.
Comments
That the bucket exists and can fulfill the objective of moving the sand and dirt is where it derives it's value. That the objective of moved dirt and sand exists is what drives up the value of empty buckets.