The new Vivaldi update personalizes your browser background with your favorite images and translates webpages faster on phone, tablets and Chromebooks.
I don't use wallpaper in mobile, but a dark theme, because it's better for my eyes. It's correct that wallpapers and screensavers are innecesary, but some people like to customize it, same as pictures or posters in your home also not really needed. On desktop almost any browser admits custom wallpapers. It's respectable if you don't like it, but anyone is free to use this feature or not, same as any other feature in Vivaldi. Use these which you want or need and hide the rest, that is the main feature.
Nah, don't get me wrong. I respect other users choices. I just didn't understand the need for something you (very) rarely see (unlike pictures on the wall).
Yes, but there are Chromiums and Chromiums. Chromium is FOSS and free customizable, gutting all the Google APIs which it has, what Vivaldi does. In Vivaldi are less Google influences as in the current Firefox.