Because some database designs can be vastly simplified if they don't need to contain tables to map to every possible type of XML document your system handles. You can extract some of the data into a tables and store the whole document in a column to refer to later.
MS SQL Server, since version 2005, has had the ability to do XPath queries on columns containing XML data. In an optimised manner. That's very handy in the average enterprise system that contains mounds of XML data in different schemas - you'd have an explosion of tables if you tried to store all that using conventional RDBMS design.
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Because some database designs can be vastly simplified if they don't need to contain tables to map to every possible type of XML document your system handles. You can extract some of the data into a tables and store the whole document in a column to refer to later.
MS SQL Server, since version 2005, has had the ability to do XPath queries on columns containing XML data. In an optimised manner. That's very handy in the average enterprise system that contains mounds of XML data in different schemas - you'd have an explosion of tables if you tried to store all that using conventional RDBMS design.