They need to be sure for many things.
They need to have product or service that will sell to their customers.
They need to be sure that potential customers will want to buy exactly their products and services.
They need to be sure that customers will want to pay the amount that will guarantee profitable operating of their future business.
The most important part in each startup is future customer and their behavior.
Because of that as an entrepreneur you will need to answer following questions before you decide to start your next business.