Functionality insists that, if you remove an item from the whole it in some way detracts from that whole. If you remove a cog from a watch, the watch may cease to function; therefore, we know the cog has a function.
Likewise, we know that if we remove a finger from a body, the body's functions become impaired, albeit slightly. This denotes that the finger has a function.
Intelligent design infers function toward a purpose, and insists that all parts of the whole further the totality of its purpose. The cogs of a watch all function toward the purpose of measuring the movement of time. The primary organs of the body all function toward the necessary processes to sustain life, reproduce, etc.