The land navigation course is traditionally conducted with a hand held magnetic compass. If you are walking the course you shoot an azimuth at a landmark and then move to it over rough terrain (In tanks magnetic compasses are useless.) . More often than not the course is designed so that you lose sight of the landmark and that will cause you to stray from the azimuth you laid out. I always hated the training that was involved because they would have you go through a swamp if there was one and into deep ravines where you could not see your landmark. The idea was to have you use your innate sense of direction.
More importantly soldiers in combat often find themselves in areas which are not thoroughly mapped at the walking level, just at the high 1:100,000, or 1:50,000 level. Getting from point A to B, or laying in indirect fires can be problematical. Having a GPS based device telling you where you are, where rounds land etc. is a huge improvement.