1) life threatening emergency medical conditions - if you are bleeding out, having a stroke or heart attack, or suffering from respiratory failure or shock, even with the best communication gear, help can be hours and maybe days away. Knowing how to perform CPR, apply tourniquets, splint wounds, perform the heimlich, treat heat exhaustion and shock, etc. can save lives. In extreme weather, even a minor injury can turn into a life threatening situation.
2) exceeding your limitations and the limits of your gear - failing to bring appropriate clothing and other equipment or attempting a strenuous hike without appropriate experience and conditioning cannot be solved by technology. Rescuers may not help you off a mountain if you are not seriously injured, if they do you may face large rescue bills after the fact, and in some states states (like New Hampshire) you may even face fines.
3) change the weather - venturing out in extreme weather conditions adds a layer of risk, since rescue teams may not be able to reach you until weather improves.