Is not required but recommended. If not set then driver displays warning message:

=================================================================  
Please ensure that you set the default write concern for the database by setting
one of the options :                    
w: (value of > -1 or the string 'majority'), where < 1 means no write acknowlegement
journal: true/false, wait for flush to journal before acknowlegement   
fsync: true/false, wait for flush to file system before acknowlegement   http://www.mongodb.org/display/DOCS/getLastError+Command                     
The default of no acknowlegement will change in the very near future       
This message will disappear when the default safe is set on the driver Db
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