The extra length keeps your lower body dry, and doesn't bunch-up under the hip belt when wearing a pack. Some water may leak in the seams in a heavy downpour, and perspiration vapor will condense on the inside, but usually this isn't a great problem. The tread should have a good grip and be long-wearing. Boots Most backpackers don't need boots since you really don't need them for trail-walking. If you plan on blazing your own somewhere, then yes, you may need a pair, along with a machete and antivenin.
Otherwise boots are too much weight for limited utility, and take too long to dry.
Most true long distance hikers on the Pacific Crest and Appalachian Trails switch to to simpler leather, canvas, or light trail shoes within a few hundred miles. Sandals Birkenstocks are popular with backpackers. In the developing world some backpackers buy locally-made sandals, which are cheap but not always available in large sizes. Others use flip-flops. Pants I travel with two pairs, usually along the line of Levi's Dockers.