You can always experiment with it.
If the whole site is done with one font color, then changing one code to #FF0000 will turn that thing red.
What if you saw something but didn't know what it meant, then you can change it to #FF0000 then check the results. If you see that thing in red, you know what it's for. 
Some websites overdo CSS coding. They put way too much in there. It's usually because the website uses a lot of formality and everything is done automatically from a database or computer center. One example would be the travel website, Expedia. 
For me, I prefer to keep the coding simple.