Big Window Clock shows the current time, in your Mac's time zone, in the largest possible font for its window.
You can use Big Window Clock to measure audio/video sync drift, where your audio and video capture devices don't run at precisely the same speed. You need to point your camera at the screen, enable the periodic "tick", and check your sync drift after an hour or so.
Or you can use Big Window Clock when you need a really big digital clock that you can read from across the room.