Starting a new business is a leap of faith. You believe in it, but your belief is probably based on a lot of guesswork and hope.
If you want something to back up your faith or if you just want to learn something about what reality really looks like and how customers actually behave, you may want to do some testing before you invest heavily in marketing, offices with designer furniture or fancy company cars.
The higher risk you take, the more you should test.
I have made an idea catalogue of ways to test your assumptions and learn more about your business opportunities: