A lot of profiles are produced by people recording tracks on a GPS receiver.
GPS is much less accurate for altitude than it is for horizontal position, and you need more accuracy for altitude to do a good job. (Show a trail wandering 500 feet from side to side over a mile, you'll hardly notice; show it wandering 500 feet up and down and that's a lot!)
And, well, elevation profiles at least tell you the general trends. You know that something looking like http://www.catskillhiker.net/maps/eaglebalsam_gra.png (just one spot that I happened to remember as being seriously tough) is going to be steep getting up on the ridge and back down off it, even if you don't know how bad the PUD's on the ridge line will be. In that particular case, not too bad, I've managed that hike in winter. I really wished I had brought crampons to climb up there, but microspikes were fine for the ridge walk. But I can tell you from experience that the little hump that graph shows at 2.2 miles isn't there. It's just the GPS wandering off. That's pretty typical of GPS tracks.