Basically what's going on is this, but with lists:

3 + 5 = X?

X = 8

3 + X = 8?

X = 5

5 + X = 8?

X = 3

In most programming languages, you can't infer a parameter by specifying a result.

(Though that's not strictly what's going on here, either)

You could further say something like X + Y = 8 and get

X = 0, Y = 8

X = 1, Y = 7

etc. for some definitions.

Which is, as the OP was getting at, sometimes incredibly useful.