Validating an idea is usually said to be the first step you should take in creating a business. You need to see whether it will actually work before you start building it, so that you are not wasting any time.
On the other hand, there is an other movement, where you build a lot of little MVPs, and see what sticks, as explained for example it the Make Book by Pieter Levels.
While validating seems indeed like a good solution, you’re not sure your product will work until it gets in the hands of the customers. However, successful pre-order sales are a very good indicator that it would, since people are actually giving you money for what you are going to offer.
Making a lot of different small projects until one works takes longer. You’ll probably have to go through it anyway if you really want to learn enough to make it work.
Your first design will be shit.
Your first landing page or pre-sales page will suck.
Your messaging will be bad.
You won’t start with a following.
Finding a solution and learning about all of those topics cannot be done in one sitting, and on each new project iteration, you will become better.
Keep building, release, then try again.