The two biggest reasons you should avoid using “Uncategorized” are fairly straightforward:

  1. It makes you seem careless, resulting in your blog appearing unprofessional
  2. It doesn’t help your readers find the content they’re looking for

When you have blog posts listed as “Uncategorized,” it simply looks like you forgot to classify the post.  Realistically, that’s the purpose of having the default category “Uncategorized” exist in the first place.  It’s meant as a catchall in case you forget to classify the post.  But remember, blog categories are meant to be useful to your readers.  Having a blog post classified as “Uncategorized” is about as unhelpful as it gets.