Example: I'm working on an PDF replacement to support marketing automation. The people looking at this website may or may not be my target audience. Thus, the feedback may or may not be useful.
You are right in that not everyone that comes to the site will be representative of your audience but "outsiders" can have their own insight into something that you may not have considered.
All the mechanisms we will be putting in place (comments, voting, project questions etc.) are designed to help you decide if your project is worth pursuing: They will not be 100% effective but they will be much more effective that creating your project in total isolation with no feedback from anyone.
These mechanisms will evolve over time, for example, we may find that straight up comments may be useless but a scoring system, where other commenters can vote some up or down, could provide far more value...
WTF is still evolving.