You have to check that the map picture on your map display is showing your correct position before descending. If the IRS positions have wandered during the flight, sure the map display is showing your position in relation to radio aids, but how do you know you are actually there? So you tune one of those radio aids in, and check the bearing on your RMI, and the DME reading, and make sure that the position on the map display corresponds with the actual radio bearing/distance from the VOR. Then you know your map is showing your accurate position.
It happens that errors can cause map shift. I have had it twice, and it is very disorienting. You suddenly can't trust what your major position display is telling you, and great care has to be taken. Now with GPS inputs to the map display, it's always bang on the ball.....but how do you really know that?