The Global Network will be used initially to flesh out all the pre-calculated options that we give you. More on it here: http://blog.askmrrobot.com/join-global-network/
We may eventually offer some kinds of rewards to people who are contributing, but we haven't completely figured out the details yet. We are tracking each user's contribution though.
If it's expensive for you to run it, then don't worry about it -- the goal of the global network is to improve the website rankings for everybody, but without increasing the cost of the website or slowing your computer down when you want to use it for yourself.