What do you think the purpose of the wall is?

The reader never knows the inital reason why the wall was built, and that is the whole premise for the speaker asking his neighbor about the purpose of the wall.  The speaker reasons, "Isn't it / Where there are cows?  But here there are no cows" (ll. 30-31) and "There where it is we do no need the wall: / He is all pine and I am apple orchard" (ll. 23-24).  The speaker does not understand why there is a wall when there is no need to keep something in place or keep something else out.  In fact, every spring, the speaker and his neighbor meet to mend the wall, and in this particular moment of the poem, the speaker wants to know the reason.  All his neighbor can answer is a saying that his own father spoker, "'Good fences make good neighbors'" (l. 27 and 45).  Perhaps the reason for the wall is simply to give the neigbors a chance to speak to one another once a year in the simple act of repairing the wall. Walls are also a way to designate what belongs to each, and perhaps this is also the reason, to prevent arguments in the future.