I work for an ISP, we run 3 server. 1 for web hosting, 1 for email, and one for establishing connections. On occasion one of these craps out and we have to replace it. If we had all the services running on one server and it went down then we would have many angry customers.
Our email server usually is always busy filtering email spam and scanning incoming email for viruses. This takes a lot of the rescources on our server.
Basically, you don't want everything going down b/c of one small problem with one part of the service you are trying to provide.
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