I was raised LDS, though I had the good fortune to spot the BS as a teenager and haven't set foot in a church in almost 20 years. I did however, participate in the baptisms for the dead when I was like 16. The theology is bafflingly stupid and cheerfully inconsiderate (a lot like Mormons, LOL, amiright? I'll be here all week!).
At least when I did it, the notion is that everyone who died without a chance to honestly consider Mormon teachings is in a kind of low-intensity purgatory rather than straight-up Hell. In this place, they will still be pestered by missionaries, but for some reason it's critical that_ someone's_ body be physically baptized in their name. Mormons are taught that they should chug along collecting their relatives names so a random teenager in Boise who lied about whether they jerk off can get dunked in an industrial bathtub perched on top of mediocre statues of oxen. This checks the appropriate box on the deceased's spiritual resume, so if they accept the teachings in the afterlife, the pre-reqs to be fully Mormonized will have been met. The current rules are only to do [edit: submit] names of your actual relatives, but yeah... Familysearch says we're all related to "Thor Odinson" and Noah.
Of course, if it turned out the Mormons were right, there are some weird knock-on effects. First, why would Earthly missionary efforts be desirable at all? You're just giving people the chance to potentially doom themselves while they are still subject to bad influences. Second, why is there no accommodation for people whose names are not recorded? Fuck them, right? Third, what on God's green Kolob is the point of pretending there is a "choice." I have likely fucked myself, being a dirty quitter (My wife once overheard her parents' Mormon neighbors describe me as an "apostate"), but if it turns out the Mormons are right, I am gonna feel pretty fuckin' dumb, and similarly, barring some "The Good Place" shenanigans, literally 100% of the people who are given the chance to accept the "Gospel" after they're already dead will accept.
Personally, I think the LDS emphasis on genealogy was mostly a way to collect intel to assert control, inflate membership in the early days, create a shared sense of history and connection in Utah where literally none of the members had deep family roots and so many had abandoned their own families for this weird culty church. Then, finally, it gives nosy grandmas a way to do God's work while pursuing their hobbies. Like so many things in religion generally and Mormonism specifically, it kind of got out of hand.