Presently, you must select a forum category before you can type out the contents of a new topic thread (the text field is uneditable until a category is selected). I think this should be changed so that you are able to type out the contents before selecting a category - that way if you accidentally press the Create Topic button by mistake before you finish typing out/editing your post, it will not create the topic precisely because a category was not yet selected. In other words, omitting the category is a safety feature against accidental topic creations while topics are still being worked on.
Yes, omitting the title accomplishes the same thing, but a “working title” mindset helps guide the creating of a post. If I can write a post without a title, why can’t I do it without a category?