In the Dutch Defense, why does white play b3 and Ba3? In the Stonewall Dutch Defense, why does white play b3 and Ba3? Should I try to avoid it, or does it not matter?

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1. Nf3 e6 2. d4 f5 3. g3 Nf6 4. Bg2 d5 5. O-O Bd6 6. b3 O-O 7. c4 c6 8. Ba3
The main reason is to exchange black's good bishop, and what would probably end up being white's bad bishop after a later e3. Another common way of doing this is Bg5xNf6. This also highlights black's dark-square weakness problem that he will have to watch out for for quite a while.

This exchange on a3 leaves black's other bad bishop on c8 more markedly bad, and the e5 square more weakened. After the exchange on a3, black can, and usually should, gain a tempo with Qe7, and the white Na3 will redirect to Nc2-e1-d4 (or sometimes via Nc2-b4-d3) to further control e5.

This scores well in my database at 67% for white, but many of the games are between players of disparate ratings. At the higher levels, it seems that black can hold equality, so I would say that if you are stronger, there is no need to avoid it, but if you are weaker against a good positional player, you might want to try to get in Qe7 earlier to make Ba3 harder, but white still has a4 then. I am not sure you can really, and totally avoid it if that is the plan white chooses.