Kodaka Hasegawa and his acquaintances have a nabe party as a drill, so that they'll know what to do when invited to a real nabe party.
Were the writers on drugs when they made this OVA? That may sound like hyperbole, but that's the genunine reponse I had when I watched this. It contributes nothing to the actual series and was not only pointless, but just... gross. Slight spoilers ahead: unless you like wasting 10 minutes to watch a literal fever dream that ends with 2 girls graphically vomiting on the MC and nothing else to happen - walk away from this one and pretend it doesn't exist. I promise you, you're missing nothing.