Tag pairs contain text as well as other elements and their content. An element declaration may also define an empty element, one that isn't designed to contain any text or other elements. Attributes provide extra information about the element. One of the rules of XML is that all attribute values, regardless of type must be enclosed in quotation marks. Attributes are not expandable.
Attributes are more difficult to manipulate by program code.
The last line defines the end of the root element. The empty platform and os elements have a slash before the greater than sign that closes the tag. Since these two elements are empty there has to be another way to provide information about the platform and the operating system. The other way is to use attributes. XML does not allow such omissions.