You can own a domain even if you don't have it attached to a website at the moment. Then the domain is parked with the registrar, but as long as you pay the domain fee, you are the legal owner of the domain.
You can own more for to ensure that other people's websites do not have the same domain name as yours.
Maybe you have a vision that it takes time to mature before you can start up your business, but you know what names you want to use. Then you can buy the domain and leave it parked, so that you have it ready the day you are ready to create a website and realize your vision.