To build a profitable business for the long-term you have to become an expert on the customer by understanding their issues, pains, desires, how they think—and for business products, how they work, and how they decide to buy.
Without this deep customer knowledge, you’re just guessing about your product's success.
Having knowledge of customers is the most difficult part of building a business. Some entrepreneurs call it front-loading the work because by carrying out a good customer research at the very beginning, you have completed 80% of the work needed for your success.
Customer understanding is difficult because it requires both qualitative learning (to understand why our users and customers behave the way they do), and quantitative learning (to understand what they are doing).