Some wallpaper apps offer features like direct downloads of wallpapers or interaction with online wallpaper galleries. To facilitate this, they might suggest making your browser the default for smoother access.
Some wallpaper applications, especially free ones, may suggest setting a specific search engine as your default. This can be a way for the developers to generate revenue through search queries.
Some wallpaper apps may come bundled with other software that attempts to modify your browser homepage or default search engine for advertising purposes.
Some wallpaper applications pull images from online sources or allow you to access a library of wallpapers from the internet. In this case, the app may need to modify your browser settings (like the default search engine or homepage) to help you access these features more easily.