Downloading styles is handy when you want to make a backup of your templates and options, or share your work with other forum administrators.
Uploading styles is handy to revert to a backup or applying the same style to several boards. You can use this option to import a style that someone has given you.
Make sure you read and understand all the options to avoid incomplete style downloads / uploads.
When you upload or download a style, the process will not transfer any image files. Your image files should be managed with an FTP client.