That's because detecting a GPS signal and thus determining your position, is a burden on your battery. Technically, this has to do with scanning the frequencies of the signals sent by the different satellites.
This is the case with all apps that use a GPS signal almost continuously. The RouteYou App or RouteYou PWA are no different in that regard: the less your position is retrieved, the less battery consumption. When navigating, however, you do not want to wait 20 seconds before your position on the map changes, because then you may have already missed the turn.