I don't think this is a good reason but tabs give you the ability to keep a bunch of files open and accessible without cluttering your main taskbar since there is only 1 Notes window.
You don't need tabs to do that. You can enable grouping on the taskbar.
I personally don't find tabs useful at all, but I don't really hate them. Well, I wouldn't hate them if more applications would avoid showing the tab bar if there was only one tab showing. I get annoyed by the waste of space that provides to those of us who will never use the tabs.
