To pray for someone to have God's blessings and God's peace only makes literal sense for someone who is alive and in the mist of the trails and tribulations of this world. After all, to be at peace in this world is almost universally, nearly intuitively, accepted as a sign of success in this life that transcends status or wealth - sometimes in spite of it.
For someone who is not alive what good is it to have someone pray for them to have peace and blessings from God?
I understand praying for God to forgive or have mercy on them, or paying debts for someone who has passed away because this could have an impact on Judgement day through God's mercy and forgiveness.
I think the answer is that it is only an action of love for the people who are responsible for faithfully relaying the bountiful message of Islam with all of its knowledge and blessings.
These questions talk about the subject of "pbuh":
These surahs are used to answer the links above:
And peace be upon him the day he was born and the day he dies and the day he is raised alive. [Quran 19:15]
Indeed, Allah confers blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [ Allah to confer] blessing upon him and ask [ Allah to grant him] peace. [Quran 33:56]
[Jesus] said, "Indeed, I am the servant of Allah . He has given me the Scripture and made me a prophet. And He has made me blessed wherever I am and has enjoined upon me prayer and zakah as long as I remain alive And [made me] dutiful to my mother, and He has not made me a wretched tyrant. And peace is on me the day I was born and the day I will die and the day I am raised alive." [Quran 19:30-33]