White 1 is very important point each other.
Moreover, lower became weak.
But, influence of white center may be easily wide,
so for black, A point is passive.
Even if white invade there, black should keep in check.
For black, next point will be black 1, so white must invade lower.
Black 1 helps white run away easily.
Black 1 is so called "knight's cap."
This white moving is heavy, and white center will become weak.
Now, white may want to block right.
If white decides to block, there is first step before it.
Commonly white can be out for weak point A.
But in this situation,
Center in danger.
Black 1 is bad.
For white 4, black stones of lower may become very weak.
For white 6, black stones of lower may become very weak as well.