Dolores' father was set up to be a significant Westworld plot point from the get-go. He was the first host in the park to glitch out after he found the photo of William's wife out in the field, and now he's holding information within him that could change everything. These Peter Abernathy Westworld theories could explain why this host is so vital to Delos.
Last season, Charlotte uploaded 35 years of data into Abernathy, hoping to smuggle it out of the park as a sort of "insurance policy" for the board of directors, ensuring Ford wouldn't take the park down with him when he was let go. So, obviously he's carrying information that forms the very foundations of the parks — coding instructions, scripts, technical details on how the whole shebang goes down. But there are plenty of more intriguing theories that speculate that's not all he's armed with. Fuel was added to that fire when Bernard took a peek at Abernathy's makeup and was shocked to see a "vastly bigger file" that he didn't seem to readily identify.
As Reddit user Nick_pj pointed out following Season 2 Episode 3, the encryption key displayed when Bernard looks through Abernathy's files looks suspiciously like the word "passenger." This season's finale is also entitled "The Passenger," as nimo90 added in the same thread. And another user, sjaano, pointed out that "Passenger" came up on the tablet as one of Abernathy's builds.
This could point to the huge theory that Abernathy is actually carrying the conscious mind of James Delos, the father-in-law of William and founder of the company currently bankrolling Westworld. Gunslinger1969 presented this theory after viewers were introduced to Delos earlier this season, and it's definitely chilling if it proves to be true. "We know Pete was an early host, we know he glitched first at the photo and was more affected by it, clearly by the photograph of his own daughter," the user speculated, adding that perhaps Westworld was meant to be Delos' "retirement world" where he could live forever. "Pete was [in an earlier narrative role] 'The Professor,' leader of a cannibal cult that preyed on others, a possible metaphor for his own corporate dealings in the real world preying on others, either for financial gain or their desire for immortality in his new paradise."
This would also explain why Charlotte chose Abernathy and not another random host to deliver the data back to the board — he could have an easier time being transported if Delos does exist inside him, and perhaps Delos is waiting to be rescued from Westworld, where he's somehow become trapped over the years.
Another theory surrounding Abernathy is that he glitched when he saw the photo of William's wife because he recognized the setting — perhaps he was one of the initial hosts used to entice investors to come to Westworld, as suggested by PullTheOtherOne, and so he's familiar with the outside world. Charlotte could be trying to get him out of the park and back into the higher-ups' hands because he has the potential to bring trouble to the mainland, because he remembers it in a way that others don't.
EvilUrges18 posed another idea that has to do with the guest DNA Bernard discovered the park is collecting — perhaps Delos is using that information to create actual clones of guests for medical purposes like organ transplants, and that's what Abernathy is carrying. This would also explain why one guest told the older version of William that his company "saved" his loved one. Flatline2962 expanded upon this theory, but made it seem as though that info could be used for more insidious purposes. "Repeated experiences plus DNA? You could build fairly convincing replications of existing people with enough interaction data," the user wrote. "This is data nobody wants to know is leaving the park- or know exists."
These are, obviously, all unconfirmed theories, and the internet is swarming with countless others. Not everyone is on board with these ideas, but you never know — even the strangest theories could prove to be true. Case in point, this post from early in Season 1, when croweschmo correctly called the fact that the photo Abernathy found would turn out to be William's, and plenty of comments shot him down. This is Westworld — no matter what the truth surrounding Abernathy's data is, you can't rule anything out just yet.