Entrepreneurs are important to the economy because they literally generate wealth. Existing companies can only redistribute the wealth and resources the company already has. By starting a new company, an entrepreneur introduces new sources of wealth into the economy, first by creating new products or services for purchase and second by providing incomes for people who need jobs.
The economy runs on wealth, which is measured in both money and in goods and services available for consumption. At any given time, there is a set amount of wealth available for distribution. This is why some people have more money than others and why certain types of goods and services are not available to everyone.