Listening to focus groups is like watching a recipe being demonstrated on a TV show. Watching customers is tasting the food so you know what went into it. (When you watch a cooking show on TV, you usually try to do exactly how they tell you to. However, if you were to taste the food, then you will make the adjustments according to what you know your guests / family will prefer).
In my view, while focus groups and other forms of qualitative and quantitative research are perhaps relevant for larger organizations who are looking at directional inputs, for startups and early-stage companies, it is important for the founders to be immersed in the user/customer’s life to understand the things that even they may not be able to articulate.
Startups usually redefine a market or sector. They usually think (or should think) of things that will do things differently than currently being done. Hence, users/consumers may or may not have a good handle on the subject as they would not have the vision of the future that the entrepreneur has painted for himself/herself.
Research is good to validate hypothesis and assumptions. e.g. to check what you think is a need gap or pain point that is really true for users/customers. Not to find if they need or want the stuff you intend to put out.