Mounting the hood in reverse is when you are storing or keeping the lens in the bag or pouch or dry cab, be it mounted on the camera or not. This is to save space, unless you have a huge bag or a huge dry cab. Then you don't have to bother with it.
I've seen many photographers shooting with the hood in reverse mount. Either they forgot that there's a lens cap or they do not want to shooot with the hood during that time or plainly thought that that's part of the lens design