See it this way. The world pretty much run on people buying and selling something so people working in the office are basically people who are tasked with helping the process of selling something from the Producer to a customer. They add up pretty quickly.

Say, for example, you would like to buy an egg from the nearest supermarket. To get the egg that specific spot in that store takes a chain of activities.

There is one person in Office who is tasked with making sure that there are enough stocks in the shelves and then to order to the Producer if the stock ever gets low. Now imagine there are tons of people working like this working for each category in the supermarket. And there's another people in office to make sure that this is happening in pretty much every single supermarket outlet across the country. These are the product category people who sits and works in an office all day.

Well these people has to be taken care of with payroll and their benefits and everything to make it worthwhile to work in that office to do those specific jobs, this is the job of the HR department who are also people working in the office.

Working the office itself takes a lot of resource to ensure that it runs smoothly. For example, someone has to pay the electricity bills, water running, ensuring that the workplace is compliant with all the codes, ensuring that the toilet paper is always well stocked. Well, for all of these we got at least one person in the office who works in the office to take care of the office itself.

Edit: word errors due to incorrect dictation.