why do use averages? are their any limitations to their use? explain by giving examples.

PCI is not the only criteria based on which a nation is to be compared or judged on. While PCI helps in comparison, it does not tell us how the wealth is distributed amongst people. 
Check pg 9 of ur economic book the table n cite that example for supporting this ans. 

We use averages because they are useful for comparing differing quantities of the same category. For example, to compute the per capita income of a country, averages have to be used because there are differences in the incomes of diverse people. However, there are limitations to the use of averages.
The limitations are :
1. It doesn't show distribution of income.
2. It also ignore other factors such as infant mortality rate, literacy rate, health care, etc...
For an example, if a country has very high per capita income then we can't say that citizen living in that country are very rich because we are not knowing about the distribution of wealth in that country. Some people might be richer while other people are very poorer in that country.

We use average for example: in calculating the money each person gets in a office. The amounts are from 1000-2000 , 2000-3000 , 3000-4000 ,. 4000-5000 and number of workers get money from the first class interval be 10, 12, 8, 3, and 2 respectively. Then the boss wants to know the average money each gets without money differentiation. Then the process to know the mean. By using mean we can take average. Number of observations divided upon total observations. Here it is 1000+2000+3000+4000+5000 / 5 = 5000+5000+5000/5 = 25000/5 = 5000. Therefore each a worker gets 5000 as an average. Average means getting equal amount of height in meters, salary, etc. to each and everyone.