What's the purpose of monitor calibration?

This was asked on a forum today, and I thought it was worth elaborating a little ... Why do we calibrate? The common, and obvious, answer is "to make our prints match our screen". Alas, it's not that simple. If we were to assemble a checklist of all the things that have to be perfect for a print to match our screen, it would look like this: [ ] Properly calibrated (and profiled) monitor [ ] Suitable lighting conditions for editing [ ] Correct use of colour spaces and ICC profiles [ ] Soft-proofing [ ] Lab properly profiled [ ] Lab regularly calibrated [ ] Suitable lighting conditions for viewing prints I'm sure I've missed some steps there, but the point is, there's a lot of ingredients in this recipe. Monitor calibration is the first and most important factor. If you're not calibrated, you can forget about reliable print matching, no matter how good the other steps are. But by the same token, a well-calibrated monitor doesn't mean anything if you don't use profiles correctly, or if your lab isn't doing things properly at their end. All the links in the chain have to be sturdy. So the real answer to the question "why do we calibrate" is "because it's an important element in making our prints match our screen".