There is something called Default Post Category in WordPress.
This Default Post Category is where the posts which you did not set any category to will end up.
If you did not configure this, the posts will be ending up in the Uncategorized category. In other words, the Uncategorized category will be the Default Post Category.
The Uncategorized category is the category which cannot be deleted but can be renamed into something else. Most users usually tend to ignore the Uncategorized category. As a result, if the user forgets to set a category for a post and publishes it, the post will end up in the Uncategorized category which looks unprofessional.
Which is why I usually recommend users to rename the Uncategorized category into a category that is often used. This way, even if the user forgets to set the category for a post, the post will most likely end up in the right category.
However, if it’s too late for you to rename the Uncategorized category into a category which you often used, there is another solution.
The solution is to actually change the Default Post Category into the category which you often used. This can be done via the WordPress Writing Settings area.
Under the Default Post Category section, select the category which you want to make as default.
And save the changes when you are done.