Firstly, the first and foremost reason is obviously because it is your wedding gown. It is the most expensive dress you'll ever own and it's part of the celebration of the most important day of your life. It is the buy wedding dresses in all of your wedding pictures. It is one of the things you'll remember most about your wedding.
Sure you have your pictures, but to actually be able to see your actual wedding dress beautifully preserved will always bring back a flood of wonderful memories.
Secondly, you may want to wear it to celebrate your 5th, or 10th or 25th wedding anniversary. You could put it on a mannequin and display it for an anniversary celebration.
Thirdly, wedding gown preservation keeps your bridesmaid wedding dresses in perfect condition so your sister or your own daughter or even granddaughter can wear it on their wedding day. (It happens more often then you may think and is a wonderful opportunity for you and the lucky girl who wears it.)
Then, many brides are making a christening dress from their wedding gown. Being able to make your wedding gown into a bridal gowns sale that your precious daughter will wear on this important day is something to look forward to. It can start a great family tradition and heirloom.
Lastly, something that is beginning to take off in popularity is making a bassinette cover made out of it. This can easily be done and provides a wonderful reminder of your special day and the special little one inside the bassinette.