you dont need to use it as first milk is ok until 1 year when they can go onto full fat cows milk. I did switch but only because it was a bit cheaper and you could get offers on it that you dont get on first milk.
Right, well, follow on milk is not necessary for babies. The NHS recommends sticking with first milk until they are 12 months and then moving on to full fat cows milk.
Follow on formula was invented by formula manufacturers to have something to advertise when advertising first milk was banned in the UK. In other countries like the US where there is no ban, follow on milks don't exist.
They have a different formulation to first milk, (based on casein not whey) and contain more protein, vit d and iron (which can cause constipation). See this a formula ? Research overview (2010)_6.pdf doc from the NCT for an explanation of the different types of formula milk.
I've never tasted formula but I imagine it tastes different due to the different composition - maybe you could buy a ready made carton and try it!
The only benefit I can see to follow on milk is that it can be included in supermarket offers and points promotions etc, so it can be cheaper than first milk.