When you have to print thousands of leaflets for a client using that client's corporate colors and, after printing, it turns out that the artwork was in RGB and the colors are not exactly those the client wanted[0], yes, you notice it. And the client always gives a crap.

Of course no one cares if you print something for your house or a friend and the color is slightly off. But in some industries something like that can cost you a big client and a lot of money.

[0] The difference can be quite dramatic, even without a professional monitor.