Index numbers were originally developed to measure changes in price levels and are used as economic barometers. There are different types of index numbers including price indexes that measure wholesale, retail, and cost of living prices, and quantity indexes that track production quantities over time. Special purpose indexes also exist for areas like imports/exports. Index numbers are important for policymaking, studying economic trends, measuring inflation, adjusting national income statistics, and helping various social scientists. However, they have limitations such as being based on samples, potential data biases, misuse, changes in economies over time, and arbitrary assigned weights.