Does the wall between the neighbors' farms serve a practical purpose? What evidence in the poem supports the view?

The wall between the neighbors' farms doesn't seem to serve a practical purpose, and the narrator tells us why in the poem.

In the poem, the narrator discusses how he meets with his neighbor every spring, in order to mend the wall that separates their property. He says that, without fail, the winter elements always cause the cleaving of the stone wall, leaving gaps in between "even two can pass abreast." Despite this, the narrator views the process of mending the wall as an "outdoor game" of sorts; he thinks that it's a waste of time to put up a wall.

To argue his point, he relates that his neighbor plants pine trees, while he plants apple trees. He cheekily proclaims that his "apple trees will never get across/ And eat the cones under his pines." Basically, since neither of them has any cattle (cows), there is no need to fear the unwitting intrusion of any animals onto private property. After all, trees are inanimate objects, as the narrator humorously points out, and cannot move themselves. Therefore, since neither neighbor has animals that can wander onto the other's property, there is no conceivable need for a wall.