The immediate point is that Black has stopped White from advancing with g4. However, it merely signals the start of a stunning queen manoeuvre, which amazed the chess world then and ever since. This is a logical response to support g4. It also weakens the square g3, which will prove to be important. 13 Bd2 should be considered.
Restraint is everything.
The queen has been a real star by reaching h7 via d7 and f5. It is now well placed to stop the freeing move g4 because hxg4 will reveal a pin on the white king. 18 g3 a5 Nimzovitch takes time out to stop White from becoming active on the queenside with a4-a5. It should ring a bell with any parent who has tried to teach a child the finer, technical points about chess, only to be met with a plea to be shown Fool's Mate again and again. I teach several chess classes out here in California, and recently one of my students brought to my attention an opening line of play that he said was popping up on the internet.