Just be make it possible or provide thousands of domains to different customers. The domain name is the identification string used in various networking contexts and it is used for application-specific naming.
There are thousands of domain names that expire every day and there should be some different reasons for expiring them. Reasons if owner forgot to renew his domain and some of them are don’t want to use domains anymore. Additionally, there is another reason that someone has moved to other projects, so they don’t need it. If you are new to the business and got expired domains or you don’t have any value.