The Windows View shows part of the current robot state, e.g. a loaded HTML page, a JSON document, etc., but also variables or attributes of certain simple types (XML, JSON and Excel) may be shown in a tab in the Windows View. When a variable is shown in the Windows View you may operate on this very much in the same way as when other documents are loaded in the Windows View, e.g. you may extract from it, test it, loop over it and in most cases also modify it.
As an example you may want to call a web service that as input takes some XML and as output also returns some XML. You may then create the input XML using an XML variable which you modify using step action that work on the content of the window showing the variable and when it has the desired form you may feed it as input to a web service step action. You may then have this web service step action store the response in another XML variable which you may then loop over and extract data from.