FreeBusy isn't a calendar. It's a scheduling assistant.
It doesn't create another calendar for you.
It lets you connect all your existing calendars.

Why?
To make your life easier.
Have separate calendars for work and family?
Are you a contractor with multiple business calendars?

When sharing your availability, FreeBusy automatically combines all linked calendars to determine when you're available. You can create events on any calendar and FreeBusy will always show your complete up-to-date availability to your audience.