Why do we use a different key to sign than to encrypt

[Guy Halford-Thompson]

But doesnt GPG generate 2 private keys (as well as public keys) when
you create a new keypair?

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only

I can understand if you use DSA and Elgamal (DSA can only sign) but
what about RSA and RSA?

On 1 March 2011 13:54, david at gbenet.com <david at gbenet.com> wrote:
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> Hello,
>
> People encrypt to your public key - you de-crypt with your private key. You sign with your
> private key. It's that simple
>
> David
>
> Guy Halford-Thompson wrote:
>> Not GPG specific, but I was wondering if someone could point me in the
>> direction of some resources that explain why we use different keys to
>> sign and encrypt (for cases where the same key _could_ do both e.g.
>> RSA).  I cant seem to pick anything up on google.
>>
>> Thanks
>>
>
> - --
> “See the sanity of the man! No gods, no angels, no demons, no body. Nothing of the kind.
> Stern, sane,every brain-cell perfect and complete even at the moment of death. No delusion.”
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-- 
Guy Halford-Thompson - http://www.cach.me/blog