They do go round our building - certainly weekly, possibly daily - with a clicker, to check how many desks are occupied. I think Tuesdays and Wednesdays are the most heavily occupied days - can't remember by how much, but we were wanting to work out which would be the best day to run an event. Fridays (unsurprisingly) are quietest - we all knew that without having it officially counted.
They have also calculated the cost per desk of running the building - building rental, taxes, electricity costs, water costs, heating, insurance, telephones, network connections, furniture costs, cleaners (not that my desk often sees a cleaner, I would say), and it's several thousand a year. If you never have more than 80% occupancy, it's quite a substantial saving if you can take up less physical space by not providing a pile of desks which won't get used.
I do think you need to have done that analysis first, though, and work out how feasible home-working is for your business, as changing could involve outlay in laptops rather than desk PCs or whatever, so it might mean you have to roll it out over a few years, if your infrastructure isn't ready for it, unless you have millions to spend.
I like the option of working from home once in a while, but I know from when I had to do it for 3 weeks in a row after writing off a car that to do it permanently would be very bad for my mental health. I live alone, and I found it very isolating, because I just didn't see other people, except for behind the shop counter if I popped out to get some milk or something.