It is used for tagging/storing all your pictures: personal, professional or otherwise. You can use the album tag like you would a photo album - e.g., an album of your vacation, your son's graduation... whatever you desire. Pictures are just another media type that MC can manage. MC can play your personal images as a slide show (slide display duration set by the user) while music is playing. Managing pictures is yet another great thing that MC does that few other music/media players are capable, and in just the last year JRiver has substantially grown those capabilities. The attached picture shows an example from my library... I have been collecting Bing's Picture of the Day for over decade now.
You can include your covers, booklets, etc, as "images" to manage, but I personally exclude those via auto-import folder rules. MC already manages music/movie/tv art independently so you don't need to double up by including them in Images (i.e., your personal pictures).
That said, you can display your CD booklets or other miscellaneous pix while playing a song (assuming they are stored in the same folder/sub-folder as the music file) by right clicking on the playing track and choosing "View Extras" from the context menu. I don't personally use "View Extras", but I am sure others that do can elaborate further.
