Does the wall between the neighbors' farms serve a practical purpose? What evidence in the poem supports your view.
In Robert Frost's poem "The Mending Wall" the speaker describes a stone wall that exists between his property and his neighbor's. We learn that the two of them annually walk along its length in the spring to repair gaps where stones have fallen out. The speaker tells us that he has in the past tried to point out that the wall is not really necessary; for instance, it is not as though his apple trees are going to reach across and "eat the cones under his pines" (line 26). But to that his neighbor responds with the phrase, "Good fences make good neighbours" (27).
The speaker ponders the practical need for walls in general, wondering if he might be able to get his neighbor to think about the matter. He knows that walls are useful if someone owns cows, but neither of the neighbors has cows to contain. The speaker also thinks that people who have walls must either be trying to wall something in or wall something else out (33). When he says that a person building a wall ought to want to know "to whom I was like to give offence" (34) by building one, he suggests that insisting on having a wall when there is no practical need for one might even cause hurt feelings.
If by "practical" you mean a property-related use such as containing animals or other goods, there is no such need mentioned. In fact, the speaker seems to feel that no wall truly needs to exist there. He is the second "something there is that doesn't love a wall" (35). He wishes that his neighbor would realize the pointlessness of having walls between them.
But practical matters aside, the neighbor does see a need for the wall. He needs it psychologically. The phrase he says twice, "Good fences make good neighbours", reveals a need for isolation. It means that a fence can prevent interaction, and thus it can help people maintain a better relationship than they might have if they risked the entanglement that freer access to one another might result in. The speaker remarks that his neighbor "moves in darkness" (41) that is "not of woods only and the shade of trees" (42), which seems to mean that the neighbor is secretive. In light of his earlier remark about wanting to know what one might be "walling in or walling out" (33), we get the idea that the neighbor is a person who avoids sharing too much of himself with others. The wall is a not only a physical representation of that wish for isolation but is a necessary tool for maintaining it. In saying that a good fence makes a good neighbor, the neighbor is actually telling our speaker, "I like it best when people give me space and do not touch things that belong to me". There seems to be no containment sort of reason that their two properties need to be divided, so the only logical conclusion we can draw here is that it makes the neighbor feel better about their relationship if there is a solid boundary in the middle of it.