If there are outliers, IQR is a better measure of the spread of data than range. The range will include the outliers, and make the data appear to have many more terms of a greater/lesser value than it has in reality. IQR shows the middle 50%, which (usually) doesn't contain outliers, so the majority of the data spread would be shown by IQR in a data sample with a significant amount of outliers. If your measure of the spread of data contains outliers, then the measure isn't accurate.