It dose have extra nutrients but its not recommended unless your baby is not eating a balanced diet and /or is not drinking enough milk for his age which is min 20 oz per day ..
If he drinks less then yes you can give him the follow on milk , but most of the time is just another money making product ..
Actually, the fallow up formula is not more or less expensive, it is the same price. It is recommended even if your baby has a balanced diet. Once baby hits the 6months mark, he has different needs.