I think uncategorized category is not a real category. It means, topics without category, just a topic holder until move it to a category. So there are some limitations what you can’t do with it.
For example:
- you cannot change the category security settings
- you cannot set up it as parent or sub category
- as @JammyDodger mentioned you cannot create an assigned chat channel from it
- maybe there are more difference…(i am not sure)
But now with the new method you can select for example your most popular and/or important category to default to make topic creation faster and more understandable. 