Root-level categories are like the foundation, exterior walls, and roof of a house. Without it, your forums would be scattered, just like you and your belongings would be without the foundations of your house. At most basic explanation, root-level categories help you organize your forums/sub-forums into categories so that your users can find them better. In example, take the "Support" category here on our community, it helps categorize the 3 support-esc forums here: https://invisioncommunity.com/forums/forum/180-support/
As you have only 1 forum there, the software would simplify that down for ease of viewing. Otherwise, the user would have to click into "Builders" to see anything, which is unnecessary .
You cannot as "Pickle" is your root-level category.