It was added to produce nicer output.

Currently:

>>> print(ast.unparse(ast.parse('a[i, j]')))

a[(i, j)]

With PR 9605:

>>> print(ast.unparse(ast.parse('a[i, j]')))

a[i, j]

The current code is not consistent with outputting parenthesis:

>>> print(ast.unparse(ast.parse('a[i:j, k]')))

a[i:j, k]

It also produces the same output for a[i:j] and a[i:j,] which have different AST and compiled to different bytecode (this is a bug).

>>> print(ast.unparse(ast.parse('a[i:j,]')))

a[i:j]