At the weddings I've been at (usually 100-150 guests), there would be families and relatives of the groom and bride, then their school friends, church friends, university friends, etc. - anything up to a dozen different groups of people who know each other but not the other guests. So a seating plan allows these friends to enjoy the party together. It doesn't prohibit them from getting to know the other guests, but offers a bit of a safe space with people the hosts know they will get along with.
(Additionally, in many cases, these groups of friends may not have seen each other for several years, e.g. since their school days, and appreciate the opportunity to catch up.)
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.
Hope that makes sense!
[1]And even within countries. I believe it varies from state to state in the US.