All Souls

Not a holy day of obligation, but everyone comes to Mass.
Why?

Maccabees

1 Cor 3

Rev 21:27

Eucharistic Prayer "... xxx ..."
which dead need our prayers?
souls in Heaven do not need our prayers
souls in Hell cannot benefit from our prayers
souls in Purgatory are the only ones left

P.S. Other scriptures
Psalm 30