You don't "need" it defaults to the shareware version if you don't provide the MPQ. I will say though that the original Diablo discs are archived on archive.org...
but talking to Farnham is not available in the shareware version.
What if I want to talk to Farnham?
Maybe I had a rough week and just need someone to talk to. Farnham seems pretty chill.
(Note: Don't torrent the archive... Apparently someone is dmca trolling that torrent. The fuck rights to a 30 year old game do they think they can enforce when you can't even fucking buy it.)