Acoustics would also struggle to determine friend from foe, if tanks or arty, and then you'd have to deal with dissipation and echoes and other weird effects. with a radar you actually can spot the the shell trajectory and turn that into a parabolic arc. then make some rough guesses as to how far they've traveled since then ("shoot and scoot"), and you can map out a firing solution that covers most of a given area.

MLRS/rocket arty is great for that since it can fire off a lot of ordinance quickly, at the cost of long reload times. saturate the grid square before their artillery can get away.

Radars get you into the EW cat-and-mouse game, tho.