The Network Location Server is a very simple requirement in the DirectAccess environment, but a very critical one. NLS is just a website; it doesn't even have to be a dedicated server, that runs only inside the corporate network. It is not publicly accessible. Every time that the DirectAccess client computers get a network connection, they query this website. If they see it, they will know that they are inside the network, and that they do not need to turn on DirectAccess. Specifically, what this does is disables the Name Resolution Policy Table (NRPT), so that the name resolution requests do not attempt to be pushed over the DirectAccess tunnels, which wouldn't exist if you were inside the office. On the flip side, if your client computer cannot validate NLS, it assumes you are out in the wild and ready to fire up that DA connection in the background, and so that process is initiated automatically.
With Server 2012 DirectAccess, there is an option...