Simply so it only changes the bits we want to, for the tutorial example it doesn't matter (as we're not using the other bits for anything) - but in a real world use some of the other bits might be used elsewhere, this way the code will still work without any changes. As you also probably noticed, I spaced the different sections apart - just to make it clearer to understand for the tutorial - by breaking it down into smaller sections you can easily look at each piece independently.
It's all explained in the datasheet, in fact page 65, table 11-4 even gives a list of various frequencies under different conditions - including 10 bit PWM, where a 20MHz clock gives a maximum frequency of 19.53KHz.