Follow-on milk contains iron in greater quantities, which can bung up some children. Follow-on milk really only exists to create a product which companies can promote, as it's illegal to advertise or promote first milk. You can earn loyalty card points on follow-on and it can be included in discounts and promotions, which again are illegal for first milk. These might be reasons for using it but there's no real nutritional advantage, and you can give first milk happily to the age of 1, when you can switch to cows' milk.