Great question — probably a blog post in itself :)
Short answer — heavy cardio workouts are like heavy workouts on your muscles. They hurt the muscles short term (heart muscle) but builds it up over the long term.
However — too much is not good! For most people.
We’re talking marathons, iron-man triathlon, chronic cardio.
Mark Sisson covers this in great detail: http://www.marksdailyapple.com/how-to-succeed-with-the-primal-blueprint/