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Import and Export blog means you are making a copy of your blog, comments and all the blog data. If you export your blog that means you are making a backup of your all the blog posts, comments and every data that exist in your blog.
Export can be done for future use of those posts, comments and other information. The extension of the exported file will be .xml
Import will help you to open those exported comments, posts and other information. If you are doing export than one day you must have to import to re-use that exported file.
In blogger, it's very simple to do import and export blogger file. For that, you must follow steps,
Go to Dashboard >> Setting >>> Other >>>> Blog Tools > Import Blog >> Export Blog