Pages saving confusion

Just struggling with getting to grips with Pages... I'm confused as to why when I open a document I have created in Pages and make a change on it, it automatically creates a new file called 'back up of' and then the file name. What I'm struggling to understand is why I can't just save the changes and overwrite the previous file, as Microsoft Word would do for example. There is no 'save' button but a 'save version' and I don't understand the difference. Please explain how this works if possible. Thanks!