Because you need to have that sign up page there before you literally write your first line of code. If you went live today you would be spending hours on an idea you don't even know people want.

Putting up a simple page like this takes half a day and you can get to know if people are interested in your world changing idea.

As an entrepreneur don't you want to know that? I do.

I'd still recommend spending a few weekends banging out a minimally-functional prototype in Rails. It's more engaging and it forces you to think through the concept in more detail. It's also good coding practice.

Wait that's absurd.

So as an entrepreneur your idea is you want to go around asking people if they'd be interested in an idea before you decide to invest time into it?

So what happens if your idea is (at least to you) pretty brilliant yet you don't get feedback. Will you drop the idea and move onto the next thing?

So with this logic I assume you have least a half dozen "idea" sites like this and whichever gets the most feedback, that's the one you will invest your time in?

Sigh.

Edit: So I just realized I started pretty much every sentence with "So"

No.

But it helps if people show interest. It is one of the many directional metric that you can use to gauge to understand what of your 10 ideas you have every week should you invest more time in.

And no, it does not mean you have a sign up page for 10 of them. :) Highly recommend Eric Ries' Lean Startup Video/Book.