In this study in the first chapter, he says always King b1
However, I don't see the threat to the King position. How does b1 help? I often see King moves just to the side that the computer STRONGLY recommends but I can rarely see the sense in it.
Its because you need to protect the a pond. It also helps with king safety.
The knight is already protecting the a pawn. is this in preparation to move the knight?
Now that I look at it yes
You "always" play Kb1 when you queenside castle because it stops a lot of threats on a2, gets your king fully safe behind the pawn wall, and avoids the potentially dangerous c1-h6 diagonal. There's never a downside to your king being on b1 vs c1.
A few commentators (GM Ben Finegold in particular, and I guess IM Rosen too) don't even explain these thoughts anymore besides saying "always play Kb1." So it's a little bit of a joke also. If you don't see anything else obvious you can do, always play Kb1!
in this game the knight is gonna kicked away easily.
the real question is why not play a3, a3 can be a weakness on the other hand its a useful move as a flight square against backrank mate threats.
i play queen side castle in my london games 90% of the time and most of the time the computer likes Kb1 rather than a3 when the a2 pawn is attacked.
when black castle king side its often a pawn race to the king for either side and with a3 it safes black one tempo to damage the pawn structure infront my king.
a3 also gives black a tactical oppoturnity to sacrifice the dark square bishop on a3 to damage the pawn structure and king safety.
i got to a point where im also always play Kb1 when a2 pawn is attacked in the london system without thinking much.