I do not understand them either. If i look at my own music files, there is much more definition (for lack of a better word) between peaks, not blocks as shown in the pictures. That looks kinda weird to me.
I mainly use spectrograms to (try

) to determine if, for example a flac file, is a converted mp3 instead of a real one. Not an exact science i know, but i noticed weird things a couple of times with flac's i bought.