I assume T23D - no one's anticipating Genisys 2!
Having said that, there are probably more anticipating that than Avatar 5, considering Cameron's only going up to 4!
Because it indicates how much audiences like and recommend the film, by giving it repeat viewings and spreading good WOM to their friends and family. This, in turn, shows how viable any future sequels will be, given the reception and legs of the first film.
It was pretty close - from wikipedia:
Biggest three-day opening weekend – £24.33 million ($36.2 million).[97] This broke the former record of £23.75 million ($35.3 million) achieved by Harry Potter and the Deathly Hallows – Part 2 in July 2011.[97]