The primary benefit of cloud services to business* is found where a business is small enough that it's more economical to outsource to a third party than attempt to run an in-house version at a small scale. The problem with evaluating the benefit, however, is that a great deal of the IT cost that can be saved are intangible.
Dollar-for-dollar, even small players can purchase 1Tb of SAN storage space cheaper than it can be had over a 3-year period with a cloud services provider. But once you strip out the time spend administering your chosen storage technology, the risks of losing disks or a staff member screwing up a configuration, power, cooling and ongoing support, the cloud offering may work out more attractive.
Also, it tends to be the case that the the parties responsible for a companies financials favour predictable interval charges to one-off unpredictable charges. This means 'renting' is more attractive than 'buying' even if buying is cheaper overall. A vendor offering you a monthly rate for X amount of disk, bandwidth, RAM, CPU or whatever is more attractive from a financial standpoint than outright purchase of expensive IT equipment (plus all the aforementioned intangible costs associated with this).
The benefit here in moving 'into the cloud' should start to become apparent to small and medium business, if their current situation is the Datacenter-in-a-broom-cupboard approach that we're probably all familiar with. Cloud providers can provide the same or similar service and do so reliably and cheaply due to the economies of scale that small businesses simply can't tap. However as you alluded in your question, companies who's business 'is IT' are often of a scale where these benefits start to be realized internally and in those cases a 'cloud provider' often can't offer as much. However, this needs to be taken on a system-by-system basis, because even a company that specializes in (for example) online data storage will probably benefit from moving their internal IT needs (email, test environments, document management) out 'to the cloud'. Their core business would of course remain in their own racks via dedicated links that they control, but they can easily remove the hassle of maintaining unrelated IT stuff (that every other company under the sun also requires) by farming it out to someone who specializes in delivering that service. This means the IT bods at that company can then spend their days focusing on their core business (in this case, their file hosting service) rather than running around wasting hours troubleshooting their company exchange server.
*Bear in mind that I work for a cloud services partner and spend my days helping clients 'move into the cloud', so my perspective on this is both informed and biased in equal measure.