folic acid is important as it reduces the risk of spinal cord problems, such as spina bifida.
As the increased intake should be taken during the first 12 or so weeks of pregnancy it is advised to take it whilst trying to conceive so as to offer further protection.
Most women may not realise that they are pregnant until 4+ week into the prgnancy so this covers them for those early weeks of development
To answer the other part of your question, folic acid is essential in cell development, particularly nerve cells and DNA formation so plenty of it is needed to start off your baby.
200mg is recommended minimum for adults, but it does no harm to take the 400mg daily recommended as minimum in pregnancy anyway.