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.