EXIF information is some special data that digital cameras store on every picture they take, such as camera model, date and time, time of exposure and other more technical details. Tools like Jpeg Exif Stamper take advantage of this information and use it to create watermarks on images.
The program is mainly intended to work with many files simultaneously, which may be the reason why we found it particularly uncomfortable to edit just one single picture. The interface is not very intuitive and when minimized, it doesn't appear on the taskbar, which makes it even more complicated.
In all, watermarks can be added in a fast and easy way when working with many files at the same time. Too bad you can only include date and time.