Many enterprises might choose to keep some applications on-premises. Perhaps they are based on nonstandard systems or out-of-date software, or perhaps they will remain on-premises while waiting for their turn to be migrated to the cloud.

In this model, some applications run in the cloud, whereas others remain on-premises, requiring a secure, high-speed communications path between the two environments.
In a way, then, the cloud becomes an extension of the existing datacenter, and vice versa.