Bands which sold 3 copies of their CD tend to have no reliable sources indicating its existance even, with no sources, how is anyone able to ensure that it even exists? (Perhaps I should of reworded that). Notability criteria in a nutshell, basically means, the subject of the article needs to be mentioned by some reliable source. If we had no notability criteria, then we simply have no ways to ensure that the subject of the article exists or even real.
Note: I'm pretty much summarizing what's on http://en.wikipedia.org/wiki/Wikipedia:N If you think I'm biased, please read the policy directly on Wikipedia itself.