As both a web and print designer, I have never seen the need to calibrate my displays. Especially for web design, I wonder why I should spend several hundred euros to calibrate when I can instead test my colour choices on multiple uncalibrated displays in different lighting conditions. Only a handful of users are going to look at my colour choices on an accurately calibrated screen while under the same lighting as I had when I stuck my ColorMunki to my screen.
For print design, I understand that you can calibrate your screen to display an RGB approximation of CMYK as accurately as possible. But it's still going to be an approximation—it's never going to be perfect because of the fundamental differences between the two colour models. After some initial surprises in my beginner period, I have gotten a gut feeling for how my work is going to come out. Especially after a test print on my desktop printer. So why calibrate?
I know that I am asking a rather fundamental question here, which many of you will want to answer with "But of course you must! Everybody does it!" Call me a rebel, but by default, I am genuinely unconvinced of doing anything that people say "everybody does it" about. Therefore, I am looking for objective reasons and arguments. Forget the feelings and the tradition, just tell me why.
Even though this question looks like it's asking what I am, it only goes into finding an affordable way, and not the why..