[veddox]

To allow different circles of friends to sit together.

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.)