Well... Banks and other financial institutions are a good example of someone that would want an "enterprise" level server. They want near 100% uptime, and multiple redundancies to prevent any data loss from hardware failure or network/power outages.
That doesn't mean that standard issue Opteron or Xeon servers can't get the job done, though. You'll just need a lot of them set up as a cluster with failover capabilities, like Google has their servers configured.