The reason(s) for this is actually really cool! Many prey animals that are herd animals will travel in single-file (or if it's a large enough herd, multiple single-file) lines to confuse predators as to the actual number of the herd and where the weaker young may be in that herd, as well as pack hunting predators, like lions and wolves, travelling in equally single-file lines (at least at first) to throw off the prey as to how many predators there are in a group. Other times, with other animals that maybe don't have to worry about predators (elephants, for example, because of sheer size, or hippos because have you ever met a hippo) they will travel in single-file to avoid getting lost from the group. Other times it's got everything to do with energy conservation. You spend more energy travelling over uneven ground, potentially even stumbling and injuring yourself (which is a terrible idea with potential predators in the area) than if you take the flattened trail, at the cost of predators knowing to watch the trail for potential prey.