Slithers is not the smartest fish in the sea, but he does have unquestioned loyalty to the boss. The boss would have liked a more competent minion, but Slithers was the only one willing to work for free. What he lacks in intelligence, coordination, reliability, and responsibility, he makes up for in flattery and adulation for the boss.
Each mastery level adds another card to the cardpool for defeating Slithers. Each of the bronze, silver, and gold mastery levels unlocks a new card, while it takes all three diamond and onyx levels to unlock a single card.