As a guest bedroom? A few times a year, when distant friends and relatives top by. As spare storage for seasonal clothes, decorations and so forth? Probably 1 out of every 4 days. As a hobby room for things that don't work out well in the office? Quite a few more days. As a sleeping room for pets (ferrets)? Every day.

Edit: To be used as a guest bedroom, we tend to reorganize a bit (such as moving the pets to the master bedroom) but primarily we consider it the "guest bedroom". It sounds a bit nicer than "arbitrary spare room".

That layout is how my house is laid out. The guest room is hardly used, but my wife works remotely, and I like to play around with side projects, so the office is quite useful.

For me personally, it's nice to have the room set aside for "work", keeping me a little less distracted.

I work from home 3 days a week, I use it for that. I'm on a lot of conference calls and need my own space with a desk and a door that closes. I also have a couch I can sit / lay on if I want to during the day, and a tv. Sometimes in the winter we watch tv in the office because it is warmer upstairs. I also use the office to store personal possessions like hobby stuff and work related plaques etc that would be taking up space somewhere else. House is 1500 sq ft and there are 2 people and 3 dogs.

In my early days, I ran my law office out of my house, and had an actual office, and clients sometimes came over. I have known a few lawyers to do this. I stopped after a few years because having kids ended up not being very compatible with this setup, but it did mean that our house was virtually spotlessly clean for a long time! My wife liked that.

For me, my "computer room" (office) has a couple bookshelves full of standard computer books, most of them are a decade or two old but still relevant. Things like W. Richard Stevens books, O'Reilly, Knuth, Tanenbaum, some books from college, etc. And I like having a table/desk to hold the model M keyboard and large monitor, and docked laptop. And I have a tower server on the side, and large 4-color laser printer.

But the most important reason for a dedicated office is to keep the cats out of my computer gear. And now that we have a toddler back in the house (S.O.'s grandkid that we are raising), keeping him out too.