[marknorton]

If you want to adjust or measure anything, you have to have some reliable reference - or proxy for it - to relate to and the closer that reference is to what you're measuring, the better the result and the higher your level of confidence. If you have a voltmeter, you rely on its readings because it was calibrated at some point against a dependable reference and you hope it hasn't drifted off since then. If you have a ruler, you rely on the people making it having done a good job in adding those markings.

We've discussed endlessly here how adjusting the camera and lens to work together might work out but might also mean you end up with a non-interchangeable lens camera - the body won't work with other lenses, the lens won't work on other bodies. Strictly, the lenses and cameras need to to be adjusted to separate, independent standards.

The advantage of an infinity test is that you can make it without letting that can of worms called the Leica M Rangefinder get in the way. Simply mount the lens, set to infinity and see if an object at infinity is in focus. The most basic focus test you can put an M body and lens to because it is made completely independently of the rangefinder.

Question is, how do you find an item at infinity? That's where the collimator comes in - it looks to the camera to be projecting an image from infinity, even though it's a short distance in front of the lens. A reflex collimator allows to you inspect the image projected back out by the camera for correct focus. Projected back out? Yes, any image relected off the film or sensor surface causes the camera to act as a weak projector and a reflex collimator allows you to inspect that return image.

So, I think an infinity test is important because it is independent of the rangefinder and establishes a reference for then calibrating it.