In the Top Gear Patagonia Special, the presenters come upon an incomplete bridge and have to construct a ramp to get their cars across. This is a great metaphor for technical communication, and, indeed, communication of all kinds: the incomplete bridge.
Technical communication is often described as a bridge between the expert and the user. But that bridge is always incomplete. The user always has to build the final span that connects the bridge to the bit of ground they are standing on.
This is true for several reasons, the most basic of which is that you have to contextualize any information you receive to your own project in order to act on it confidently and successfully. If the document tells you to push the red button, it is still your job to determine if you are looking at the right device and the right red button, and if your purpose will truly be served by pressing the red button at this time. The document can never entirely ensure that you do not press the wrong button on the wrong device or at the wrong time for the wrong purpose. Only the individual reader can determine those things, and thus only the reader can build the final span of the bridge.