The reason for this?
Weddings bring together people from all parts of our life and many of them have never met and probably will not meet again, however they are all there to celebrate the couple getting married and have a connection with that couple.
This usually means people will want to see and share photos of the wedding, and that usually happens online these days.
With many wedding photographers taking weeks to return photos, some (not all) couples will be happy for guests to share photos that they have taken.
The problem with many social media sites, and the settings that most people have in place on Facebook, is that any photos that they have will only be shared with people that they are friends with on Facebook, or Instagram, meaning that other guests will not see these photos.
Hashtags (#) create something akin to an album that people can view if they know the name of the album. The name being the chosen hashtag, this allows all wedding guests to view the album and share their pictures not only with the couple but with other guests.
If there are particular photos that you are in or like, you should tag yourself in them so that they appear in your newsfeed as well and you can easily find them, in the future. If you want to print them, usually you can download photos and print them.