Prospecting is important for a sales person because it is how a sales person can get new customers. Prospecting is typically defined as the process of finding potential new customers. This is, of course, of the greatest importance for a sales person.
The whole point of sales is to sell more of your firm's goods or services. To increase sales, it is generally necessary to find more customers who are willing to buy. In order to find new customers, prospecting is necessary.
Business in a free market is a process of constant competition. If one firm or one salesperson does not engage in prospecting, they will likely fall behind those who do. It is always necessary for a sales person to search out new prospective customers so as to maintain the firm's health.