The sages say (Shab. 88a) that at that moment the ministering angels gave each Israelite two crowns, one for Na’aseh and one for Nishma.
The people’s two promises, doing and listening, represent the two aspects of Jewish practice – practical observance and intellectual understanding.
It is interesting to see which one comes first – the performance or the perception.
It is not that one can manage without the other. Merely delving into the mitzvot with one’s mind might take so long and be hedged around with so many qualifications that it never led to practical results.
Merely performing the practices inherited from family tradition (“I keep a kosher home because my mother did”) might become so routine and perfunctory that one’s mind never harnessed itself to the action.
The two aspects both need to be present. But carrying out the commandments takes precedence.
It trains a person to be a Jew. It gives one the psychological feeling that the mitzvot are right for one’s life.
It creates the emotional climate that makes one delight in the law (in Psalm 1, the tzaddik is the person whose delight is in the Almighty’s Torah and by adopting and observing it he makes it his own).
That is why the sages put in the mouth of God the words, “Let them forget Me – but keep My commandments”.
It’s not that God really wants to be forgotten, but by keeping the commandments we have a sure way of finding Him.