Why should I book my workplace in advance? I could just go to the office and find an available desk there.

Well, that is possible, of course. But when you work at a company that has less workplaces than employees, there is a chance that there are no desks available, when you arrive at the office. When you book a desk in advance, you are certain you have a desk to work on. Especially if you prefer to work on a specific floor, a specific desk or near a specific colleague.