The interface to the shared-use datacenter, if you're lucky, is a spreadsheet that declares the static resources you own and a remote hands guy that can tackle things beyond the capabilities of your remote KVM. If you need more capacity you need to work with the datacenter folks to order physical machines that might show up in a few months.

The interface to the in-house cloud is an API. In most instances, developers are completely abstracted away from the physical infrastructure and don't need to take a lock on some human in the datacenter to get their work done.