This isn’t to keyword stuff in the image filename & alt text, which is often useless at best.
Google doesn’t index CSS background images. Furthermore, without Open Graph markup for a specific image, social media platforms won’t grab a unique background image.
Instead, both often just grab the first HTML-based image found instead, perhaps something in your header, so the image is the same for all pages or nothing is shown at all.
Therefore, your pages will lose appeal when listed on search engines & social media. Having a relevant, high-quality HTML image for that page through a mobile thumbnail image on Google or next to your link on Facebook means higher clickthrough rates.
Finally, if you have unique images you want to rank, never put them in the background! (You can also do a lot more here, and place it above the fold, to ensure your images have the greatest impact.)