What a thing is for. Purpose is assigned rather than discovered, which is why a thing can be repurposed.
Old French porpos (an intention, a plan), from Latin proponere (to set forth, to propose): pro- (forward) + ponere (to place); a purpose is what a thing was placed forward to do, and can therefore be placed forward again.
What a thing is for, assigned rather than discovered. The root's act of placing is reversible by nature — what was set forward for one end can be set forward again for another, which is exactly what repurposing does.