If you are not validating, you are guessing.
Gut instinct, experience, and all the rest of it, can have a place, as long as that place is below data and facts.
Validate at every opportunity to stay aligned with the needs of your target audience.
The basic premise of validation is to acquire data to ascertain if an idea has potential, and if so, how much.
It is used to test if you have correctly identified a problem or pain point for a particular target audience, and if your solution addresses it. You will also need to ascertain if you have a decent market size, and if your target audience is willing to pay for the solution.
Validate in the simplest, most efficient, most accurate way possible. This sounds very utopian, for it is. You will need to run multiple tests to get nearer to the truth, so things like landing pages, surveys, whatever it takes to get inside your potential customers heads.