In the natural law tradition of such thinkers as John Locke, our personal privacy is one of the fundamental rights that we are entitled to as human beings. It is important because it allows us a space to act in the way that we want to, unhampered by government regulations.
On a philosophical level, this is important for the sake of our free will. If we live in a totalitarian society, we are in some way deprived of our free will. We cannot choose to act and believe as we will. This takes away an important aspect of our humanity.
On a more pragmatic level, personal privacy is important because it reduces the scope of government. Life in a police state, in the sort of place where there is no privacy, is harsh. It is the life that Orwell writes about in 1984. This is not something that we would ever want to live with, which is why it is important to maintain our personal privacy rights against the government.