Do you walk around every single day with your umbrella open? You know, just in case it rains. Probably not. (but hey, if you do, you do you)
Perhaps you check the forecast so that you can be prepared if it rains. Maybe you don't, but you do have an umbrella in your car, your briefcase, your backpack, or your purse.
The point is, your umbrella is a tool you put to use when it's raining, when you need it, when it serves a purpose. This is one way to look at an office.
An office is a resource. A tool. A space, if/when needed, can help employees in a variety of ways. However, it is not always necessary. Just like the rain determines the need for an umbrella, the work is what needs to determine the need for the office.
Side note: some people actually enjoy walking or running in the rain without an umbrella. Go figure. One person's need for a resource is not necessarily everyone's.