Vitamin drops are recommended for all babies, BF from 6 months and FF from 1. The reason is for the vit D. Most childrens vitamins also come with A,C,and E in them too but those aren't the focus of the recommendation, it's all about the vit D.
We get little vit D from our food but our bodies produce it when exposed to sunlight. When it's miserable weather we have lots of clothes and coats etc on when we're outside and therefore our skin is not exposed to sunlight. When it is sunny we use sunblock, stay in the shade and still cover up, particularly our babies. So we're not absorbing it then either.
There have a been quite few cases of rickets in recent years and this is what has prompted the recommendation.
It is just that though, a recommendation, it is your choice whether you choose to follow it or not. Your HV should have explained why it is recommended or otherwise it can give the message that formula is better than breast and breast is lacking. It isn't, we know that.
If you know spend lots of time outside, with your skin exposed and you eat a good healthy diet with lots of food that contains vit d, i.e fatty fish, eggs (yolks in particular), milk and cereals fortified with vitamins (Just some examples) then you'll probably have enough stores to pass onto to your baby through your milk. The darker your skin the more exposure to sunlight you'll need.
The HV is just passing on the recommendation from the DOH, it is her job, but she should always qualify the recommendation with the reason why, which is unfortunately lacking too often.