This main window button changes the way the attribute list is presented. Normally LEX shows only the attributes which are currently filled with values. This is because objects can potentially have a lot of attributes and they are often more clearly represented when you don't see the complete list but only the ones with values. However, you can see the list of all attributes if you deactivate this button.
As you can see the unused attribute rows are displayed as gray lines. To show or hide the attribute which currently have no values, you could also use the menu option View - Attribute Output Settings - Show Only Attributes That Have Values.
When you deactivate the Show Only Attributes That Have Values button to show the complete list, and select one of the empty attributes, you can easily set a value to it. Just double-click it, or press F2 or ENTER to edit its value:
Please remember that the attribute display could also be filtered by the attribute filter configuration. The attribute filter specifies generally what attributes LEX shows in the list, with the Show Only Attributes That Have Values button you configure in addition to that if the empty values are visible or not. So both settings are mixed together.