Why a subkey?

[Mike - EMAIL IGNORED]

On FC4 with gpg 1.4.1:

I created a new user account and used gpg --gen-key .
I selected RSA (sign only) since it was the only RSA
option.

It says the key cannot be used for encryption, and a
subkey must be generated. Why?  Is it related to
(sign only)?  If so, why was (sign and encrypt)
not offered as an option?

I did this a year or two ago, and I do not remember
needing a subkey.  I still have that keyring in
under another user.

Thanks for your help.
Mike.