Notability is important for preventing a potentially slippery slope towards Wikipedia being expected to have an article on every shop, every street, every apartment complex, every popular teacher, and every creative work ever appreciated by more than 10 people.
What is wrong with that?
Sources. There are no secondary, independent sources about every shop, street, apartment complex, popular teacher, creative work, or the fact that there is a pencil lying on my desk right now. No matter how true it is, it is not verifiable in any reasonable sense of the word.
This is what people don't understand when they complain that things are deleted from Wikipedia. If Wikipedia's ambition is to create a credible encyclopaedia of all human knowledge, then it cannot be filled with speculation and half-truths. Even primary sources are suspect. I could easily create a blog or web site that claims something, then create a Wikipedia article that uses my web page as the main source. THAT is the slippery slope that is so often talked about.