Make both ends strong
Charles Matthews There is some common advice on frameworks like this:
Here is better than a play like Black a. After both black groups have a base so that Black can fight strongly when White invades. Black at a prevents White from making a really easy invasion: but it seems to do less that is definite and Black doesn't yet have much territory.
John Fairbairn: Don't you think the proverb play away from thickness covers it? And if we apply principles as opposed to personal style, wouldn't be better on the fourth line? If so, "balance" is an applicable principle. I can see also a case for the principle of inviting the opponent to invade your framework (a would not be a framework).
BobMcGuigan: conforms to the principle of strengthening the weaker stone when extending. A play at a instead would be over-concentrated relative to the corner. It would also allow a good white invasion below the star point on the upper side. After the invading stone becomes stronger there could be aji against the black corner as well.
Bill: Black is already overconcentrated in the top left corner. At first I thought there was a small low enclosure there, but it is a strange one space extension from the 3-3. Was there an original typo in the diagram?
I'd also argue that this sequence available to White is a good reason not to play at b, instead.
Charles: Yes, that does look strong. So in this case might be close to an overplay.
This is an uncommon side pattern. The reason would be that the enclosure with BC would normally take a lower priority amongst big points in the opening. It was played in relation with black strength in the lower right.