Have you ever tried to read the combat log? Maybe you're on the PTR testing a spell change or you want to know how often a new trinket procs, so you open the combat log chat window and start reading.
You get twenty-something lines down before you realize something horrifying -- that's only one second of combat. Your jaw slowly falls to the floor and you reach up to pull your hair out as you contemplate reading a combat log for an entire raid fight -- that can sometimes last more than six hundred seconds and includes far more actors than just you and the training dummy.
Luckily for us, when events are printed in a specified format without too much variety, computers can read and process --or parse -- these lines magnitudes faster than we humans can. World of Logs has graphs and charts to help us understand our performance, but it also includes what is basically Google for your combat log. Just like there are tips to speed up and pinpoint your browser searches, there are tricks you can use to query events better in the World of Logs expression editor.