With private roads its necessary to maintain the road and make a profit for the owners of it, without tolls (or some other way of raising revenue) the service wouldn't be provided. It's also a fairer system as those who use the service actually pay, rather than having all taxpayers put in to it, even if they don't own a car or ever use the roads.
The toll money that is paid goes to pay for repairs on the road. That's what I've been told and it makes sense. You're basically paying for the wear you make on the road. That's pretty much my argument. 11 words left holy cow now only 4! 3 to go! And done.
Toll roads are necessary so states can raise revenue for the repair and upgrade of the very same roads. States must balance their revenue sources, and one way to do that is to collect tolls from roads and major bridges. New York City takes in billions of dollars in revenue every year from bridge and tunnel tolls that have become a regular way of life in America's largest city (by population). Revenue from the tolls goes towards the road itself, both for repairs and for adding lanes. Transportation used to be a trademark for America. With many highways in dire need of repairs, more states should have toll roads to collect money for highway projects.