He needs the manifest to remind the others of their 'constant' in the way that Charlie showed him Penny as his constant...Here is a very good theory I ran across:
"There is a connection between love and constants. How souls go beyond any dimensions in time. Desmond's constant is Penny, and his mission is to find the Oceanic passengers in the alt who still remain on the island in the that timeline, and help them find their constants. He will trigger their memories by reconstructing a key event from their island lives, and by helping them find their constant. In this episode we see that Charlie and Daniel begin to remember through their constants, Claire and Charlotte. I BET YOU in the next few weeks we will see Desmond identify who's still left on the island while in that reality, and then find them in the alt when he flashes back. Through finding their constants, their on island selves will begin to have "memories" or visions of the alt, and see what their lives would be if they prevent MIB from leaving the island, by sinking it! Here is a breakdown:
In the alt:
Hurley: Desmond will introduce/remind him of Libby, his constant
Sawyer: His constant will be Juliet
Charlie: Claire, already seen
Daniel: Charlotte, already seen
Claire: Charlie
etc..
On the island:
Hurley: Will see his life in the alt as lucky, his love Libby will be alive, maybe he ends up with her!
Claire: She gets to raise Aaron
Jin & Sun: Je Yeon survives in the alt, they can be a family together
Jack: He gets to be a dad
Sawyer: See's himself meeting and ending up with Juliet
etc..
When the on island Losties see what they can have if the island is sunk, they are willing to sacrifice their lives to sink it! This is it!! If MIB leaves the island, the release of electromagnetism will be catastrophic. The world will end, and everyone they love will cease to exist. The alt visions give the Losties the incentive and willingness to sacrifice their lives to save the world! They sink the island to prevent MIB from leaving the island, and prevent the release of the electromagnetism.
There is only one end, and this is it! While there is a new candidate, they will never take over Jacob's spot. Think of it as Neo in the Matrix.. he's the "one", selected for the specific purpose of "saving the world" and thus resetting the Matrix, but he never fulfills that role, as his LOVE for that girl was so strong. I believe Jack is the candidate, but he will never take over as protector of the island, he will destroy it.. there is only ONE END.
Purpose of Jacob and MIB. Jacob is literally the jail warden, making sure MIB doesn't escape, causing the end of mankind. His candidates are replacement jail wardens, to do the same job as him.. UNLESS, the candidates are willing to make the ULTIMATE sacrifice and give their lives to destroy (sink) the island, preventing MIB from leaving. He needed to find someone who was willing to give up their desires, wants, needs.. someone who wasn't power hungry, corrupt.. those would be the only people who would be willing to sacrifice their lives for a greater good!! As MIB pointed out in the conversation with Jacob, that hadn't happen yet, but the difference this time.. DESMOND!!! By giving them the ability to peer into their alternative lives, to jump between realities and see through the looking glass, he is able to reassure them that there is something good waiting for them on the other side, so dropping their own desires in the on island timeline is achievable."