The spiritual implications are interesting but there are also some that could be materialistic.
Some people have the the ability to do date calculations, ie they can tell you what day of the week any particular date in the past was. I wonder if any of the people who can remember every day of their past calculate the day of the week rather than remember it. Maybe they would use the same ability for dates before their birth or in the future. I don't have an opinion about it, I just think is is just a hypothesis worth testing.
And how far back do their memories go? Do they start on the day of their birth? What about when they were still in the womb?