For me the novel description is the first glance for any potential readers to get to know about the writer's writing style, writer's skill level, the tone conveyed, and a glimpse of the story. I can understand why it is very important (although I surely did not make a good impression with mine). Cover and genre alone does not suffice for me because it is too vague and it did not reflect the writer's skill, you cannot judge someone's writing skill by their graphic design or drawing ability.
For example, when I am faced with a long and unnecessarily detailed description, I cannot help but think the actual novel will be the same (and I have proven right in some cases). Usually, reading a description takes less time and effort than reading a whole full first chapter, that is why I would prefer relying on my impression of the description rather than first chapters. Description also can let me know if the story will potentially contain contents I dislike, or maybe help warning people of content that may trigger them or something.
That is why I feel very cheated when some books sold in bookstore do not even have a small blurb in the back. I noped away no matter how interesting the cover is. I do not want accidentally investing my time and money into things I won't like, and I am sure this is how many people think when reading a description to decide if they're going to read or not.