How is the classification of economic activities into primary, secondary and tertiary useful?

On the basis of classification we are able to analyse the pattern of total production and employment India. Classification based on different sectors provides us information about the contribution of these sectors in the Indian economy and employment provided by these sectors, also howover they earth ere contribution in Indian economy is changing. During the development phase of an economy, the contribution and employment of the primary sector decrease and increase in there maining two as people move out of agriculture too the sectors where more employment opportunities are available.