The Strava route builder incorporates the popularity of other users routes when you plan your own. So, in an extension to the normal find-shortest-path algorithm between two points, penalties are given to paths not used by other users or bonus given to those that are popular. The result is that you end up with the better roads to ride on. Especially on the bike this is really useful as I can plan new routes in unknown areas and they are pretty good.

For running I found it remarkable how these heatmaps show the inclination to run along water bodies or in forests. It can almost perfect predict where I would or wouldn't run.