What whois said isn't wrong, but also there's SUCH a downside...
The service is usually tedious.
You're bound to be sat next to someone you don't know at the meal.
You'll have to eat and drink to their agenda and menu, so expect to starve/be dying of thirst and then get served something you don't especially like/isn't very nice.
Unless you're "in" (a family member or close friend) you won't know most people there. And even if you are "in" you're only likely to be "in" on one element of their lives. (I.e. if you're a school friend, there's bound to be loads from Uni. If you're a cousin you know family stuff, but none of school or higher education stories. If you know the groom, you prob don't know the bride and vice-versa)
You have to have "an outfit". You have to provide a gift. (money, money, money). They might have a collection as well. And the bar is rarely free. And you have to get there and home again, or maybe accommodation (all spends!)
You have to sit through the "hilarious" speeches, which will only mean anything to the "in" lot. (and only ever some of them, some of the time)
If you're like me, you'll be wearing underwear which will start to chafe and make dancing a fantasy you'll never be able to partake in. And shoes hurt!
The photographs are interminable, and you're going to look like shit in all of them.
You have to slap on a smile and be positive about EVERYTHING to EVERYONE.
I've been to some lovely weddings, but for the most part, IMO: Make the most of watching Strictly with a bag of crisps in your pjs!