If it's a processing issue, you need not buy more storage now because your servers are untethered. If you need more storage, you can simply buy more storage, as opposed to having to buy more servers. The separation of server from storage also makes it easier to add storage later, when needed. A study by the Gartner Group indicates that the average organisation's current storage needs are growing at a phenomenal rate, and doubling each year. The current methodology for managing and protecting data don't scale well.
SANs allow for scaling without proportionate increases in staff.
Let's consider an analogy from a human resources perspective. A single human being, working full time, can perhaps manage 200 gigabytes of storage. This is possible because the SAN concentrates storage in its own network where it is easier to manage than if spread across a variety of networks. With adequate system and path redundancy, SANs do not have any one point of failure. This, along with easier and more powerful backup and disaster recovery, are ensuring that SANs change the way enterprises think and how they deploy their servers.