It's not to make more people see your pictures, its so attendees can find/share each others' wedding photos with one another. It's like a Gmail label so the next day I can click on the hashtag (that everyone else in the world doesn't know/care about) and easily see all the pictures guests took while they were there.

Yeah so the reason for a wedding hashtag is so you can easily find all the pictures that people take with their phones. So it's a hashtag that the guests are given so when they take a picture of the wedding or reception, you can find it later. It isn't a tactic for making your wedding "viral" or anything, it's a label…