Well, for one thing. It will remind you there is more than one way to skin a cat. As my grandpa always said.
For another thing. It will re-introduce a very deep sense of reality to your (often abstract) planning. Two people are joining their lives together. Forever and ever, Amen. And even if the chargers were the wrong color AND the flowers were the wrong color AND the photographer was malicious AND one of your groomsmen backed out 9 days before the wedding AND every single person who hugged you managed to dig the veil deeper into your scalp… Even then. It’s more than a day, more than an event, more than a never-ending list of chores. And sometimes it takes being at a wedding other than your own to bring that back into focus.