They know from their own experience that we need help, inspiration, assistance on the journey. And since they love us, they are eager to assist us. They are eager to help us, and the only way they can do this is through prayer. Since God is love, all who are with God love God and love their fellow creatures whom God also loves. Love is never self-centered.
Love is always concerned about the other.
Love will help where help is needed. Now nobody needs help in heaven, but we need help here on earth. Therefore the Saints will love their still-struggling brothers and sisters, will assist them with their prayers. She does not command it - that we pray to the Saints for their intercession at the throne of God.