Why is this move better?

Why Ng4 was better than g6 ?

This is the standard Dragon setup. The best attacking plan for white, after the bishop is fianchettoed, includes the exchange dark square bishops, which would weaken the dark squares. By playing Ng4, black tries to secure his DSB and have a safer king. Due to this plan, white normally plays f3 or Be2 first.

Most teachers will show an example where there's an exchange of a white rook for the black bishop on d4, to demonstrate how weak those squares become.

Thank you for explaining the move . If you could tell me good way to learn openings, it would be very helpful
@jonesmh

The best way to learn why moves are played is to have an experienced player explain it.

https://b-ok.cc/book/659193/cbf5a4 Chess Openings Explained, Explained.
https://b-ok.cc/book/1290677/0c06cb Starting Out: The Accelerated Dragon...

Both are good books at explaining the moves of the Accelerated Dragon. If I remember, neither discuss why f3 is played, they only praise the black moves. Starting on page 63 of the first book, they show a variation where Ng4 is allowed and give, "black has a comfortable game".

A better, but much harder, way is to find the answers yourself. (The more effort you put in, the more likely that you'll remember.) At chessgames.com. searching under the "collection" tab, you can find what others found interesting and they often have good comments (after you read through all he garbage).
https://www.chessgames.com/perl/chessgame?gid=1067858
https://www.chessgames.com/perl/chessgame?gid=1067831
(Just a couple of games that show the weakness of the dark squares.)

If you don't want to buy books, you can also watch the free videos of Saint Louis Chess Club, iChess and agadmator on youtube. Agadmator is best.
@Bishop00