"PBKDF2" begins by "PB" which means "Password-Based". This is for passwords. A password is not just a sequence of letters; it is a sequence of letters that a human can remember and type. A 256-bit key, generated as keys should be (with a cryptographically strong PRNG), is not a password, even if you encode it as letters.
Another way to say it is that the core of the functionality of PBKDF2 is to cope with the inherent weakness of passwords: since they can be managed by human brains, they are weak and can be brute forced. A proper cryptographic key (like the one you generate with
RNGCryptoServiceProvider) does not have this weakness, and thus has no need for PBKDF2.
Now there is a tangential use: PBKDF2 is not only PB, it also is a KDF. It has a configurable output length. As such, if your source key is shorter than what you need, then a KDF, in particular PBKDF2, can be handy to obtain as much key material as you wish. For instance, you have a 256-bit key, but you need to encrypt and to have checked integrity, and for that you would like to have a 256-bit AES key and a 256-bit key for HMAC. To turn a 256-bit master key into 512 bits of key material, you need a KDF. However, PBKDF2, with its password-specific idiosyncrasies (input is letters, not bits; there is a need for a salt), is ill-suited in general for such jobs. SSL/TLS uses for such jobs a custom function (which has been quite thoroughly investigated, since SSL is a high-profile target for cryptographers) which it calls "the PRF".