Is it really important to have an audience BEFORE launching something?

Just wanted to get you guy's opinion on this. Usual tried and tested way to successfully launch something is to build an audience first depending upon their interest and then launching the product.

Now, if it was like this where you had the product ready but you know that the audience is already there - how would you reach out to them?

I'm working on this short guide about a video game that I think might get some people interested - If I were to launch it now - how would YOU guys approach the marketing aspect of it ? - considering I havent done any prior audience/community building like having twitter engagement or something like that.