(a) Tertiary sector relates to activities that help in the development of the primary and secondary sectors. These activities by themselves do not produce a good but they are an aid or a support for the production process.
(b) The causes for tertiary sector becoming important sector in India by replacing the primary sector are as follows :
(i) Concept of welfare state and basic services : In a modern welfare state such as India, the government is supposed to provide basic services such as hospitals, educational institutions, defense, posts and telegraphs for the welfare of the people. These functions come under the tertiary sector.
(ii) Development of agriculture and industry : With the introduction of new agricultural tools, the production increased. At the same time invention of machines led to the development of industrial sector. These developments were responsible for increase in demand for services in tertiary sector such as transport, banking etc. Thus, development in primary and secondary sectors led to the importance of tertiary sector.
(iii) Rise in income: As the income of the people rose, there was more demand for services such as, public schools, shops, private hospitals etc., for better facilities.
(iv) Development of information technology : The development of information and communication technology has increased the importance of tertiary sector because now most of the people want to avail these services such as mobile phone, internet cafe etc. More and more people are entering these services. Thus, the tertiary sector has become most important sector in India.