This move looks quite logical. Rather than shutting this bishop away with 5…e6, black develops it actively. This bishop can later swing around to help defend the king, with …Bh5 and …Bg6.
So why doesn’t everyone play this move with black? The idea doesn’t come without its downsides. White can sometimes gain a tempo or two by attacking this bishop, aiding their kingside expansion, and now this bishop won’t be able to support black’s queenside breakthrough with …c4 later on.
That said, if I had the black pieces here, this is probably the option I would choose – letting another piece defend the kingside is quite valuable!
Which Move Order to Choose?
So far, we’ve been looking at playing the King’s Indian Attack by starting the game with 1. Nf3
This has its merits – by avoiding early pawn tension, white makes sure that they’ll be able to play this set-up no matter what black does. White also immediately takes the e5 square under control, preventing black from occupying the center with their King’s Pawn.
That said, beginning the game with 1. e4 has its merits too.
The latter half of the video above is dedicated to this “Move order discussion,” but for a quick example, let’s consider the position after 1. e4 c5 2. Nf3: