Reason 1  

One reason is that becomes a good move, pressing Black low. Then White can look forward to the bad aji at the attachments at a and b, as well as the 3-3 invasion at c.

Reason 2  

Another reason is that when the outside White stones are strong enough, then White can look forward to attaching at and then playing at the 3-3 point^[1]. White can look forward for reducing Black's territory or increasing White's influence, or both.

Ladder relationship  

Specifically, the ladder after should favour White in order to play the , combo.

Not possible to live  

In normal situations, it's impossible for White to invade the 3-3 point directly. This sequence is the most common variation to kill the White group.

(However, White does have the choice of fighting instead of trying to live in the corner-- see 4-4 point low approach low extension tenuki, diagonal attachment, 3-3 invasion for more details.)

(5d analysis by Minue)

Correct plays(locally speaking) for black are showed in following 2 diagrams.

Common play of black. (1-1)  

The intent of is to defend B's corner securely. In this diagram, , moves might(may?) be put off, depending on a condition of whole board situation.

Common play of black. (2-1)  

Another way to play is ,which is somewhat modest. In this case, it is often seen for w to invade at 3.3 immediately.

Common play of black. (2-2)  

Of course, this is most peaceful sequence.

Evaluation with KataGo

4-4 point diagonal attachment, low corner enclosure  

This is the basic diagram. There are several questions to answer here:

• Is significantly worse than a?
• How does a pincer stone at B affect the matter?

The evaluation is done using KataGo as a reference.

No pincer stone

All star point opening  

In a symmetrical parallel all star point opening

• is 0.2 worse than invading the corner (-0.9 vs -0.7)
• The diagonal attachment reverts back to a 0.7 lead as opposed to the preferred move, a 3-3 invasion in the opposite corner
• is the blue move
• loses 0.2 with respect to a, the blue move

With a pincer stone

San ren sei  

In case of a Black san ren sei, White's lead is 1.1 to start with. Again, 3-3 invasions of either Black corner are preferred.

An inside approach at loses 0.6 and we start at 0.5. This is our reference for the discussion.

• This time the diagonal attachment of and White's response are both blue moves.
• loses 0.4 while a maintains the 0.5 score. and follow.

San ren sei  

Interestingly, putting pressure on the white stones may prompt White to treat those stones lightly and approach at . Things can become quite heated from that point onward.

With a supporting stone on the side

Outside approach  

Suppose this time that Black has played a (suboptimal) move on the side . White's approach on the outside will result in this sequence. Traditionally it was considered bad for Black to kick at but nowadays, due to AI evaluation, making White stronger towards an open side is considered playable because Black gets sente.

If instead Black plays low, he'd lose about 0.7 in this position:

Outside approach  

In this case White's stones are considered more resilient, allowing White to take sente. Both a and b are now available.

About the 5d recommendation

Gote  

KataGo doesn't like all that much. makes a base. Black can indeed gain influence towards with and but this doesn't offset White's sente.