Sorry...I missed the second part of your question in my answer above. If you click http://www.nail-solutions.co.uk/Nail%20anatomy.htm
a link will take you to a website all about fingernails. Once there, scroll down to the appropriate heading and you will see just what the lunulae are rather than what they are for.
If i remember rightly no-one actually knows what they are for but they are little pockets of air....they must serve some purpose (or why would good ol' mother nature given us them) but as yet no reason has been found.
Just a small point: they are not always visible, although they may exist nearer the pure root. Some people only have them on their thumbs, if at all. I am not certain, but I think they may be less prominent among certain races.
they are called lunula or also known as the half moon they are under the nail plate and near the nail bed they help to protect the nail plate and to help from diseases .They look white because of the reduced blood supply.