Heatmaps show you where users click and what attracts their attention, they don't show you WHY though. User testing, on the other hand, shows you WHY they get attracted, what they expect when they click, and what confuses them.
Based on
Nielsen Norman Group study, 5 users
reveal about 80% of the problems. “Adding more users increases costs, yet the number of
findings quicly reaches the point of diminishing returns”.