why a just opened AI file need to be saved before closing without doing any changes?

I am working in a very regular AI file, it have no variables or any external links. each time I open that file in illustrator, I see an asterisk beside its name tab in my workspace, which mean the file is unsaved and need to click on save button to get rid of that asterisk. it is not a big issue specially the file open with no problem. but why it happened?