I still dont get why you should never build a product first then market it after

Lets imagine I have 20 hours in total to allocate for a given project, goal at the end of the 20h is to decide if its worth continuing or not. I will handle the project myself – will code, market etc myself.

I see 2 options outlined

Option1 (which I do and everyone says you should not do): Spend 10h building it then spend the next 10h market it, see if there is traction and tweak the product based on your first feedback.

Option2 (which is the recommended way): Spend 10h engage with people, make them join a waitlist, get requirements from them, build the product.

I think its hard to convey your ideas without building some sort of a product (could be a high fidelity dynamic wireframe), and I dont see why as a user, I would engage in a community for something that does not exist.

Is option2 whole idea that if you involve potential users from the start, they will love your product even more and will market it for you?