There are going to be so many screenings of the movie at so many theaters that getting a ticket to the movie isn’t going to be a problem, but if you want to be sure you’re at a particular showing at a particular theater, or that you get the best seat, assuming you’re looking at a reserved seat theater, then you’re not going to wait around.
While breaking the record for the first hour might not seem like a lot, it shows just how many people aren’t simply planning on being there when Endgame opens, but want to make absolutely sure they are.