Well, remember that a certificate certifies something: The CA signs that "somebody came to me with the public part of a private/public key pair, and I have verified that this person controls (domain), so it is safe to use that private/public key pair for encrypted communication with (domain)".
For various reasons, you can have multiple servers handling (domain), though.
Now you either have the same private/public key pair for each of those, or you don't. If you have the same key pair, then if one of your servers is, for example, physically stolen, you need to change that key on all your servers. (Also, you need to transport the private key from one server to the others in the first place. Since the private key needs to be secret and well-protected, this can be difficult.)
On the other hand, if you have a distinct key pair for each server, you just need to revoke the one certificate for your stolen server and life goes on.
But this requires that your CA will certify multiple valid key pairs for the same domain, and that's a duplicate SSL certificate. (And you might agree that "I want the user to trust that this, too, is (domain)" could make you suspicious. If the CA revokes any previous certificates for the domain, at least the connections to the first server will break quite visibly.)