An appliance does one or possibly two jobs. Typical appliances are ADSL routers or a hardware firewall. In smaller networks, these devices are mostly there to keep your servers simple and dedicated to the job they've been bought to do: although that isn't really about performance, it's about intricacy. The more intricate the configuration of a server, the less easy it is to predict how it will perform under stress: and here we assume that small businesses stress their servers more than large businesses do.

Appliances reduce the intricacy of servers, by doing simple jobs with simple kit. And that's how you get your users to believe that the network is more resilient.

Some people try to get by on appliances alone, or make their devices do more than they're sold to do. This is hard to justify unless you're in a medium-sized business, with a bit more time to play around and take the risks.