Well, for me, perhaps it is just the environment i like most about cool edit. Once you have your finished mixdown in front of you in cool edit, the editing is very intuitive. You can cut out sections (for example the beginning or end crap you don't want), add silence to the parts that shouldn't have any sound, use noise reduction (probably the best tool), use a limitor / compressor to smooth things out, etc. I like the sonitus eq better in sonar though.
One example where i just recently used cool edit. We did a cover song which had a serious tempo change half way though. Insted of using the tempo change-a-ma-bob in sonar, i just did two different sonar projects, exported both to cool edit and spliced them together.
Cool edit also has a spectrum analyzer so you can have a look at your music in a different way.
Cool edit has an mp3 converter which does not run out, so if your sonar mp3 converter expires you have another option.
I'm sure many of the things i have mentioned can be done in sonar, this is just the workflow that suites me best. I found a program that lets me run cool edit inside of sonar, through the tools menu so the programs work side by side which is nice.
One final thing, i don't just use cool edit for mastering. I also use it to clean up my indiviaual tracks before i start the mixing (i.e. amplify, edit out the crap, add silence to parts etc.)