The move a3 serves basically two functions:

1. White can develop the bishop to c4, and keep it aimed on d5, even if Black attacks it with b5, or, as Giri did, with Be6. In the lines where Black plays ...e5, the battle for d5 is often very important so it could be worth spending a tempo in it.

2. Black often plays b5 to gain some space on the queenside. The move a3 then discourages a further advance of the pawn to b4.

Another reason to choose this line could be that the main line is heavily analyzed, beyond the 20th move, and the White players didn't want to engage in a theoretical battle. This would especially apply to Carlsen who often plays 'sidelines', and this tweet from Karjakin confirms it as well:

The only reason why I played 6.a3 against @anishgiri is that I forgot adaptor for my laptop and I urgently had to create a new ideas😆😆😆

_{Thanks @DagOskarMadsen and @DavidRicherby for drawing my attention.}

The move a3 serves basically two functions:

1. White can develop the bishop to c4, and keep it aimed on d5, even if Black attacks it with b5, or, as Giri did, with Be6. In the lines where Black plays ...e5, the battle for d5 is often very important so it could be worth spending a tempo in it.

2. Black often plays b5 to gain some space on the queenside. The move a3 then discourages a further advance of the pawn to b4.

Another reason to choose this line could be that the main line is heavily analyzed, beyond the 20th move, and the White players didn't want to engage in a theoretical battle. This would especially apply to Carlsen who often plays 'sidelines', and this tweet from Karjakin confirms it as well:

The only reason why I played 6.a3 against @anishgiri is that I forgot adaptor for my laptop and I urgently had to create a new ideas😆😆😆

_{Thanks @DagOskarMadsen and @DavidRicherby for drawing my attention to this tweet.}

The move a3 serves basically two functions:

1. White can develop the bishop to c4, and keep it aimed on d5, even if Black attacks it with b5, or, as Giri did, with Be6. In the lines where Black plays ...e5, the battle for d5 is often very important so it could be worth spending a tempo in it.

2. Black often plays b5 to gain some space on the queenside. The move a3 then discourages a further advance of the pawn to b4.

Another reason to choose this line could be that the main line is heavily analyzed, beyond the 20th move, and the White players didn't want to engage in a theoretical battle. This would especially apply to Carlsen who often plays 'sidelines'.

The move a3 serves basically two functions:

1. White can develop the bishop to c4, and keep it aimed on d5, even if Black attacks it with b5, or, as Giri did, with Be6. In the lines where Black plays ...e5, the battle for d5 is often very important so it could be worth spending a tempo in it.

2. Black often plays b5 to gain some space on the queenside. The move a3 then discourages a further advance of the pawn to b4.

Another reason to choose this line could be that the main line is heavily analyzed, beyond the 20th move, and the White players didn't want to engage in a theoretical battle. This would especially apply to Carlsen who often plays 'sidelines', and this tweet from Karjakin confirms it as well:

The only reason why I played 6.a3 against @anishgiri is that I forgot adaptor for my laptop and I urgently had to create a new ideas😆😆😆

_{Thanks @DagOskarMadsen and @DavidRicherby for drawing my attention.}