The extension doesn't actually reads, writes or changes any of your data, but it needs that permission because:

One of the extension's features is displaying a tooltip when manually re-sizing the window. The UI for this is created by adding some HTML, CSS & JavaScript code to all the pages a user visits. In order to do that, the extension needs access to modify a page's DOM (if you're not a developer, just know that the DOM represents the internals of a page), but, if it can access everything in the DOM (the data a page contains, the forms you submit or your browsing history on that tab) it doesn't mean it actually does read it or does anything with it. It just needs to access the body of a page and append a new element to it.