Funnily enough I am completely the opposite way round.
I always wanted to start writing the code first, but then started realising that was a waste of time if there was no-one to sell it to.
So now I always start with trying to find customers, via a simple sign-up page or similar, before I even think about writing code.
Not saying that either approach is right, but interesting that we've come at it from opposite angles, and both ended up changing our opinions completely!
I come from this path too. I've usually found that writing code is too easy. Confronting the potential customers is harder to do and I believe the hard part of the business.
IMHO
It probably depends on what exactly the idea is, but yeah I think what you are saying has a lot of value. I never made a sign up page. It was like, "I'm going to build the next reddit.com" kind of ideas. I don't think a sign up page would get you anywhere with something like that.
I always wanted to start writing the code first, but then started realising that was a waste of time if there was no-one to sell it to.
So now I always start with trying to find customers, via a simple sign-up page or similar, before I even think about writing code.
Not saying that either approach is right, but interesting that we've come at it from opposite angles, and both ended up changing our opinions completely!