For example, you can use the graveyard mechanic to replenish mana reserves, but this only applies to demons, heroes, and magic units that do not have a combat spell. Or, in one turn, transfer the hero's graveyard to the castle using units lined up in a chain. This is useful for "softening" a neutral unit, pulling the graveyard out from under the unit, transferring it to the castle, and then attacking with an army led by a living hero, or continuing to "finish off" the neutral army (for example, for resources or an artifact).
In general, there are so many different ways to use the graveyard that no post would be enough to cover them all!