I cannot remember all times I've used the reason, but here are a few:

1. Aircraft reporting on-time, yet flight tracker app indicates inbound aircraft still has not left departing city. Use this information to catch different route so that I can make all my connections home.
2. Connecting flight delayed for maintenance. Last or second last connection to my final destination, so good time to scurry over to the other gate and beg to get on the on-time flight instead.
3. Flight delayed for "miscellaneous" reasons. If this is the only flight by this carrier today, quickly start booking with a different carrier in case flight is cancelled outright. I have 24 hours to cancel new booking.

Being 1 or 2 clicks away from all this information would be great to have. Currently, I am swapping between 3-4 different flight tracking apps to see everything. Maybe I'm just obsessive this way and other flyers don't follow their flights as intimately as I do.