Most of us believe that outdoor air is healthier than indoor air. It would seem sensible, then, to air out your crawl space. It's so damp down there - letting in some air should solve that problem. And since you're never actually down there, you might as well air it out all the time. Problem solved? Not exactly.....
Sometimes the solution is worse than the problem.
And sometimes the solution is not a solution, and worse than the problem.
That's what venting a crawl space is.