There are several situations where report is really useful when submitting a petition. One possibility I can think of is a recently posted thread regarding a presumed bot sending a list of commands/directions to a player in tells accidentally; in this situation a /report of the text would very quickly locate the text in question for any investigating GM.
If you're going to use the /report command, make sure you scroll up to the relevant text before doing it. It will send in the last twenty lines of whatever you have up in your text window, of all your open windows. Close any private ones before sending the report if you have personal info you don't want sent.
If you have a larger block of text to report than twenty lines, you can send multiple reports; just scroll to the next relevant bit and rinse repeat. If you use /report <offending player name> it helps to speed up the process of investigation for the GM involved. You should also follow up any report command with a petition with more details of what you witnessed.
Hope that helps
Edit: It's 20 lines, not 100 btw.