So you have to be careful when you submit those colors in your images especially if you've added color in the yellow or cyan range. SWOP standard stands for Standard Web-Offset Press. When the standards were set up in the '70s there were a lot of printers who had very old equipment that they couldn't calibrate easily, so they settled on a standard that would apply to everybody. There is a movement now to reduce that and put the standard under more control. In my shop we use an IT8 SWOP target that we calibrate every month in a process called close-up calibration.
Tonal compression is another major print factor.
There are about 17 million different colors that can be detected by various media, but when you go to the press, it can only reproduce about 1.2 million of those colors. That compression will take place regardless of the starting material. So while one could question the wisdom of spending time to give your publisher or the printer the best resolution you can manage, the reason for doing this is to gather as much information to go to the printing process. That means dot gain on press, mechanical dot gain - the product and mechanical effects on press. The physical dot gain is the result between the ink font solution and the paper.