The lie angle of irons needs to be properly fit for each golfer for two major reasons. First, the lie angle is a factor that affects the accuracy of the shot. The lie angle is considered to be perfect for the golfer when the sole arrives at impact perfectly parallel to the ground.
If the lie angle is incorrect for the golfer, such that the toe of the clubhead is tilted well up in relation to the heel, the face is automatically pointing to the hook side of the target line. Conversely, if the clubhead arrives at impact tilted with the heel well up compared to the toe, the face of the clubhead is pointing to the slice side of the target line.
The greater the loft on the face of the clubhead, the more off-line the face will point when the lie angle is not correct for the golfer. Thus, the lie angle is much more important to be fit to the golfer in the irons than it is in the woods, since irons have a little to a lot more loft than do woods.
Second, golfers come in all varieties of height and arm length. There is no possible way that all golfers can play their best with the one standard lie angle that is designed on all ironheads. Golfers who buy their clubs from a custom clubmaker will typically be fit for the proper lie angle on their irons as a normal part of the custom fitting session that custom clubmakers offer. Golfers who buy their clubs standard, off-the-rack, without being fit for lie angle are risking accuracy problems in the form of pulled or pushed shots even when they make a good swing.