Since he said "use" and not "practical use" or "lucrative use" or even "productive use", I'll answer for myself: I really enjoy passing time making fermi estimates and back-of-the-envelope calculations (sans envelope).

For example, any time I go to a new restaurant, I try to work out the annual profits based on the time of day, cost of a dish, number of people, staff, location, etc. I don't want to open a restaurant myself, but it's fun to try to think of all the inputs I can, and do some arithmetic in my head to get an answer. It's an enjoyable way to make time pass while waiting for my meal to arrive. For me, it beats reading Twitter.