The key to why the exposure compensation is needed for bird photography for example, relates to what you said: "to fine tune the metering on the fly for a faked reading to tell the Auto ISO not to use the setting it suggests but a new value."
The shutter speed and aperture can't be used to adjust the exposure compensation as these have to stay within range to get a focused shot of a subject that can be moving quickly within and across the frame.
The 'faked reading' is really just a bias that moves the histogram to the right to brighten up a small dark area, or darker details in the subject which is a bird that you have no control over. Therefore you only may get one chance for a shot or sequence of shots under poor conditions. Poor conditions could be bright back lighting, shadowing of the subject by some object, the subject distance making it relatively small compared to the whole frame.
The main thing being, that you need to bias the exposure to to overexpose some or most of the scene in order to get this one subject exposed well enough to bring out the detail you want.
Keep in mind that for this type of subject, the exact species of bird may not even be able to be identified if all you can't make out the details properly.
With practice a bird photographer can tell how much bias is needed for a starter setting that will increase the chances that the exposure of the subject will be within the ball park in the few seconds or less of the opportunity to take the shot, then post processing will be more successful, or even possible to get a useable result later.
