In Inferno, the first part of a famous poem, Cerberus guards Gluttony (the third circle of Hell). It torments the gluttons.
Oda took some inspiration from this poem when creating Impel Down, such as Level 3 being Starvation Hell.
Anyway, it's ironic that Shamrock's Cerberus is used to help the gluttons (the Celestial Dragons).
He will likely play a major role in potentially ending the starvation tactic.