I got GotG3 advanced tickets the day they became available, and by the time I checked for seating on the Thursday night premiere several theaters and times were mostly sold out. And this is a multiplex that had it going in 4 to 5 theaters.Which seems silly because odds are you will be one of a handful in the theater. I went to GOTG3 during an 8pm show on the first day and it was only half full. Of course the theater had a showing every half hour over four rooms. Unless you really want a certain seat at a certain time at a certain small theater, there is no reason to buy in advance.
If I don't get advanced tickets, I don't get to watch the movie on opening night in good seats at a reasonable time.