This fullpage image + small amount of text is a fad it will pass just as scrolling text went away in the 90s and "intro" pages when away in 00s (god i feel old)

In this case the background image adds absolutely nothing to the user experience or conveys what the content is about, if anything its quite an example of bad design and interferes with the user for no good reason! The "web developer" has added it just because he could not because there is any good and thought out reason to use this "image page monstrocity"