Last year on KAW I fell onto a rock, and split the skin on my shin. I did a reasonable job of cleaning it with fresh water from my bottle by the side of the trail, and dressed it at my b&b that evening from my FAK. It still went manky however, and required a course of antibiotics. My FAK now includes iodine soaked gauze.
Generally the rest of the kit is for minor crap like blisters, small cuts etc, but also includes a trauma bandage just in case, perhaps even for use on others. The most useful course i ever did was a Hostile Environment Course, where the first aid part was entirely focused on major trauma, stopping somebody dying before a competent person can get to them