Image from Jerusalem, Plate 8, Pulling Moon
Generation is one of the two fallen levels of existence in Blake's poetry. On page 49 of William Blake's Circle of Destiny, Milton O Percival explains:
"The two levels of existence in Eternity become four when the soul fails to maintain even the lower paradisiacal level. This occurs when man, doubtful of energy and proud of repose, tries to perpetuate his repose. With this negation of energy, spiritual sleep deepens into spiritual death. The infinite universe shrinks into the finite. The ethereal body, hardening into flesh becomes a tomb for the imprisoned spirit. This is the world of Ulro. But man is not permitted to perish utterly. A way of salvation is provided. A shaft of light pierces the tomb. Energy reappears and struggles for release from the temporal and the finite.This is the world of Generation. Eden and Beulah, the two unfallen levels, together with Ulro and Generation, the two fallen levels constitute Blake's four worlds. They represent four states of the soul, corresponding to four degrees of spiritual vision."
Generation as the field of man's redemption runs thorough Blake's poetry although he rarely explicitly states it. When you look for it you can see it.
Milton, PLATE 41 , (E 142)
"These are the Sexual Garments, the Abomination of Desolation
Hiding the Human lineaments as with an Ark & Curtains
Which Jesus rent: & now shall wholly purge away with Fire
Till Generation is swallowd up in Regeneration.
Then trembled the Virgin Ololon & replyd in clouds of despair
Is this our Femin[in]e Portion the Six-fold Miltonic Female
Terribly this Portion trembles before thee O awful Man
Altho' our Human Power can sustain the severe contentions
Of Friendship, our Sexual cannot: but flies into the Ulro.
Hence arose all our terrors in Eternity! & now remembrance
Returns upon us! are we Contraries O Milton, Thou & I
O Immortal! how were we led to War the Wars of Death
Is this the Void Outside of Existence, which if enterd into
PLATE 42 
Becomes a Womb? & is this the Death Couch of Albion
Thou goest to Eternal Death & all must go with thee
So saying, the Virgin divided Six-fold & with a shriek
Dolorous that ran thro all Creation a Double Six-fold Wonder!
Away from Ololon she divided & fled into the depths
Of Miltons Shadow as a Dove upon the stormy Sea."
Think about eating the fruit of the tree of knowledge of good and evil. Was that simple disobedience or the beginning of the incarnation? Was the Biblical Eden more like Blake's Beulah or Blake's Eternity?
The purpose of Generation is delineated in the liturgy of Catholic and Anglican Easter services. The segment of the liturgy named 'Exsultet' includes these words:
"This is the night
when Jesus Christ broke the chains of death
and rose triumphant from the grave.
What good would life have been to us,
had Christ not come as our Redeemer?
Father, how wonderful your care for us!
How boundless your merciful love!
To ransom a slave you gave away your Son.
O happy fault,
O necessary sin of Adam,
which gained for us so great a Redeemer!"