In the poem "Mending Wall," does the wall between the neighbors' farms serve a practical purpose? What evidence in the poem supports this view?

The practical purpose of the wall is to serve as a divider between the properties. 

The speaker points out that walls are normally used to keep livestock enclosed.  Since neither of the neighbors have livestock, there is no need for a wall according to his point of view. However, his neighbor wants to keep the wall in good shape because he believes that “good fences make good neighbors.” In other words, he believes that a little separation between neighbors is a good thing.

He only says, "Good fences make good neighbours."

Spring is the mischief in me, and I wonder

If I could put a notion in his head:

"Why do they make good neighbours?"

The neighbor is repeating his father’s saying, and probably his father’s ritual. He wants to meet his neighbor once a year to repair the wall. The speaker finds it a bit amusing. “Something there is that doesn't love a wall,” he says, because the wall is coming down. It is just regular erosion and wear and tear. However, they have to meet and fix it. 

He moves in darkness as it seems to me,

Not of woods only and the shade of trees.

He will not go behind his father's saying,

And he likes having thought of it so well

He says again, "Good fences make good neighbours."

The practical purpose of the wall then is to keep a barrier between the neighbors and to get them together once in awhile to fix the wall. Even if they are not particularly chatty while they are doing it, it gives them a reason to interact and collaborate. In working together on the common project, they become better neighbors. However, according to the neighbor, they should then go their separate ways, and the distance and separation between them should make each of them happier.