Weddings are usually somewhat formal. Formal events benefit from guidance to reassure guests. For example, a dress code helps you narrow down your choices if you have no idea what to wear. Similarly, a seating plan is great for someone that is there on their own and has no idea who they can sit next to.

The trick is to combine guidance with a relaxed atmosphere where it's fine to ignore it. Your dress code might say "black tie", but it's great if someone turns up in jeans because that's what makes them comfortable.