It’s not really a rule


     It's not really a rule of thumb that you MUST use an external recorder.  In many cases, recording audio directly to your camera using an external microphone connected to the camera can be OK as well.  But there are some things to consider.

     For one, you mentioned it in your post that camera position can be part of the reason.  Using an external recording device, you can get the microphone closer to your subject for clearer, cleaner audio.

     Also, a lot of cameras, although they may have an input for an external microphone, may not have any way to visually monitor the audio levels.  Even if you have a jack to plug in a pair of headphones to listen to your audio, it's not the same, especially if your camera has the ability to control the volume of the sound going out to those headphones.  In other words, what you hear in the headphones may not necessarily be what is being recorded to the video.  Most external audio recorders will be able to give you a visual representation of your audio levels so that you can see if the signal is too low, or if it may be peaking too high.

     Another reason may be that you would want to use your audio recorder to record vocals, while you use the in-camera audio to record ambient sound.  This, of course would depend on what it is that you're shooting.

     I'm sure that there may be other reasons that other videographers would give, but those would be my reasons.  Hope this helps.

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