Game 2 WCH Blitz 6. a4?

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paragoncd

I was reviewing the games played yesterday.  Carlsen often plays Italian which is very instructive for me since I do as well.  I don't quite understand the move of 6. a4.  I know this is not uncommon in this line, but I don't really understand the move.  Could someone help clarify the aim of the move.

 

My thanks,

Para 

paragoncd

Thank you, always instructive!  Thank you for the continuation instruction also.