The speaker in Robert Frost's "Mending Wall" finds the wall that separates his property from his neighbor's to be extremely unpractical. It doesn't keep hunters from crossing their land in pursuit of rabbits. It doesn't keep the cows from wandering from one field to the other because neither neighbor has livestock. As the speaker sardonically remarks, it's not as if his apple trees are going to march onto his neighbor's land and eat up all his pine cones nor are his neighbor's pines going to cross into the speaker's orchard. The speaker directly states, "There where it is we do not need the wall," proving that from his point of view, the wall has no essential purpose. Even nature "doesn't love" the wall and seeks to tear it down each winter and spring.
What the wall does provide is a lot of work for both men, but at the same time, it creates a reason for them to interact with each other. It results in "just another kind of out-door game" played by the two men who probably would never take time from their schedules to play a game with each other. The wall's purpose, though not practical in the physical sense, is practical in the relational and emotional sense. The neighbor likes having the wall. It allows him to fulfill his father's longstanding, if inapplicable, aphorism, "Good fences make good neighbors." In the case of these two men, perhaps, their bad fence, the wall that always needs mending, helps make them good neighbors because it gets them together once a year and keeps the speaker's neighbor happy.
The wall, then, doesn't have the traditional practical purpose one might expect a wall to have, but it does perform a valuable function in the case of these two neighbors.