I don't know the full history of it, but I think it's basically the chance for people to object to the marriage (I guess it weeds out people who are already married?). It also went up in 2014/15 from 15 days to 28 so that the Home Office had more time to investigate potential sham marriages/civil partnerships (eurggggghhhhh).

I have an excellent memory for some things! Where I left my keys? Nope! Things that happened in friends' lives? Yep!