Why seperate signing and encryption keys?

[Christian Hammers]

Hello list

Can anybody explain me why it is quite common to have two seperate keys
bundled to "the secret key" where one (e.g. DSA) is used just for signing
and the other (e.g. ElGamal) just for encryption.

I ask because on a netlaw mailing list someone told that in Austria the
new email-signature laws would prohibit using the same key for signing and
encryption of emails (when they should be used as official documents).

thanks,

 -christian-