Self-Mill decks deliberately put cards from their own library into the graveyard. This quickly stocks graveyard resources for reanimation, flashback, escape, descend, recursion, and other effects that care about cards below the battlefield.
Muldrotha, the Gravetide can replay useful permanents that were milled, while Living Death can turn a stocked graveyard into an army. Laboratory Maniac and Thassa's Oracle can even transform an empty library into the objective and win condition.