Charlotte, Daniel and Miles all have previous histories involving the island. Frank was probably previously on the island in the Dharma Initiative
Frank flies planes and helicopters. It's likely that if he worked for Dharma he'd be a pilot
Frank is not on the Island in 2004 and knows nothing about what's going on there. Depending on how long he's been away from the island, he may not know that the Purge happened. It's likely that he's been continuing his duty as a supply dropper, just as Kelvin Inman continued his duty of pushing the button.
Frank is the only major character left from season 4 without his own flashback episode.
Manifest Memorization
Frank memorized the manifest so that when he arrived on the Island, he would know if he met a survivor of Flight 815.
Lapidus
Frank memorized the manifest because he was guilty about Seth Norris crashing the plane where-as Frank feels that he could have made it to L.A.
Frank is On Charles Widmore's and Eloise Hawking's "Team"
Frank says to Sun "they say they're the good guys... I know, I'm not buying it either." This line is significant. Either, the producers are telling us that we shouldn't assume that Ilana and Bram (and incidentally, Jacob) are "the good guys" just because they say they are, or, Frank is undercover and actually on the side of Charles Widmore/Eloise Hawking.
This could be the case because (1) he flew Widmore's helicopter to drop the freighter crew onto the island and (2) he was somehow substituted as the pilot on the Ajira flight to fly the Oceanic 6 back to the Island. Oddly, he always seems to be the guy who flies people into the island- an island that shouln't be that easy to fly into.
Widmore and Hawking aren't necessarily the same "Team"
Frank was arranged to be on Flight 316 by Widmore, possibly via Sun. This would give rise to the idea that Frank and Sun have left together on one of the three outriggers on the beach near the Hydra Station where they landed. These two are likely plotting against Ben on behalf of Widmore.
Frank said he makes the run to Guam all the time.
Candidacy
The reason Frank didn't fly is because he overslept that day.
He is a candidate to become a member of the group Ilana/Bram belong to.
Ilana knows that John is dead, so she thinks Frank could be a Candidate for the new Leadership role of the others. Frank was supposed to fly 815, but he didnt. When the monster found out the pilot wasn't Frank, it became angry and killed Seth Norris, because it knew that someone else would become the new leader of the others instead of Frank. That someone is Locke.
Just a pilot but essential for the escape of the Man in Black
Pilot Lapidus
Frank Lapidus is the only one who might be able to start the Ajira plane and take off the island, after the plane has been turned around.
He is thus essential for the physical escape of the Man in Black. Ilana is fully aware of that, which is the reason why she makes sure that Frank stays with her group so the Man in Black won't be able to get off the island.
If that were true the Man in Black shows an alarming, irresponsible lack of interest in Frank Lapidus
Or Frank is not the one necesary to later help the Man in Black get off the island but Ilana and her companions
All things considered, that plane is not a viable means of transportation. If it was, Widmore's group would have dismantled it.
The Last Recruit
Flock's goal is to fly everyone off the plane. This will require a pilot. Lapidus is the Last Recruit (the last person needed by Flocke). The episode on 4/20 will be Lapidus-centric.
Frank is not the only remaining survivor who knows how to fly a plane. At some point Jack 'took a few flying lessons' but it 'wasn't for' him. (Pilot, Part 1) He may know enough to fly the plane off the island if it came down to it.
A "few flying lessons" hardly qualifies Jack as being able to fly a commercial airliner off a hastily-built run-way. We know that it wasn't long enough to accommodate the planes landing.
The last recruit is Frank but in reference to being Jacob's last recruit. Frank is going to have been talked to by Jacob, but has never been talked to by The man in black. This is essential to his ability to kill him, just as it was with Jacob. Ben was able to kill Jacob because he never let him speak to him, and every other character has been spoken to (and decided to go talk to) the man in black. They are, as Claire put it, "with him" - except Frank.
Ben DID let Jacob speak to him beneath the statue: Jacob tells Ben that he has a choice on whether or not to kill him.
Frank is Immortal, God
Frank was the only character not shown in the flash sideways world
There was a religious statue that looked suspiciously like Frank in this world
Frank is one of the few main-cast to not have a character-centric episode, as a result his history and background are almost completely unknown
Frank survived the submarine explosion, it was not shown how. He was miraculously found drifting in the water completely unharmed.
He drifted in the water for an undetermined period of time without food or water.
Frank was able to fix the crashed Ajira plane, in violation of natural laws. At one point, he cleared the cockpit of Richard and Miles so that no one could witness what acts he performed there.
Frank survived a helicopter crash somehow. When we first see him he's merely found in the middle of nowhere, not even near the helicopter he supposedly landed
He survived crashing the helicopter in the ocean and expertly managed to fly a helicopter moments before the freighter exploded
He never really has a confrontation/conversation with the man in black, while the man in black attempts to convert everyone else to his side
Jacob's bodyguards think he's necessary somehow so they don't kill him, but knock him unconscious and bring him along with them. They also trust him with the secret of Locke's corpse
Despite all the situations he gets in, he never has more damage than a few scrapes
Frank is the one who's with them the two times they're able to escape the island, the second time he's the one who directly gets them off
Frank's physical appearance also has drawn comparisons to Zeus and Jesus/ It seems that Frank Lapidus could be a divine entity
Frank knew everyone that was on Flight 815, it's said he memorized the passenger list, but it could be his divine knowledge
Everyone on the plan that Frank flew away was in the flash sideways world, except for Frank. Whether the plane crashed, landed, or whatever. Frank clearly did not die at any point in the future, even after Hurley and Ben had.
Frank's contempt of women ("So much for the fairer sex, eh?") may explain the many sudden deaths inflicted on female characters.
