Why the gallery has to be shared to work
A private upload box collects photos from the sort of guest who fills in forms. A gallery that every guest can see collects photos from everyone, because the moment somebody sees themselves in the grid they go looking through their own camera roll for something to add. Between the main course and the speeches, that loop is doing more work than any reminder you could print.
It also gives the room something to do. Guests at table eleven who have not met the ones at table three end the night having seen the same photographs, and the couple — who were in the middle of it and saw none of it — get the evening from twelve different vantage points instead of one.
Shared does not mean open. The gallery exists at an address nobody can guess, it is blocked from search engines, and only the people you handed the code to ever see it. What sits behind the private host page is different again: that is where the event is managed, individual uploads are removed and the whole gallery is downloaded.