Most cameras and smartphones number their images in a predictable format. For example, iPhones use the format IMG_0001.jpg, IMG_0002.jpg, etc. If you upload IMG_0345.jpg to a post and publish it and then upload IMG_0346.jpg to a post that you are not yet ready to publish, someone could guess the URL of IMG_0346.jpg and see it before you’re ready.
The Unpredictable Image Filenames plugin renames image files when you upload them to a sufficiently unguessable name. For example, IMG_0345.jpg could be renamed 334AB1E8-28AB-4BE1-882D-3C112E95F055.jpg, and IMG_0346.jpg could be renamed A67C9CF9-0BB5-4FB4-AD03-DCB294F853EC.jpg. Try and guess that!