The Psalm for Thursday ends with the phrase, "I will feed with fat of grain and satisfy with honey from a rock." Why is the word "rock" included? What does the word "rock" add to this sentence? How would the sentence be different in meaning if the word rock were omitted?
Rashi on Tehillim 81:17 explains
and… from a rock: He sated them with honey when they went in His ways, as the matter that is stated (Deut. 32:13): “He suckled them with honey from the rock.”
This is part of the analogy in the Torah as to the blessings of fertility given to the land of Israel.
He made them ride upon the high places of the earth, that they would eat the produce of the field. He let them suck honey from a rock, and oil from the mighty part of the crag.
As Rashi explains
He let them suck honey from a rock: It once happened in [a place in Israel called] Sichni, that a man said to his son, “Bring me pressed figs from that barrel.” The son went [to the barrel, but instead of finding pressed figs,] he found honey flowing over its brim. The son retorted, “But this is [a barrel] of honey [not figs]!” His father responded, “Dig your hand deep into the barrel, and you will bring up pressed figs from it!” [Pressed figs are as hard as a rock. Thus, we have an illustration in the Land of Israel of “sucking honey from a rock.”]- [Sifrei 32:13]
Rav Hirsch explains that
Under the special blessing which Hashem granted the land of His Torah for the People of the Torah, the fertility of exceeded the ordinary natural nature of the soil, stony and gravelly sandy soil changed to orchard soil and bore dates and olives. See remarks on Gen. XXVI,1,2
Note that the full analogy to special fertility (by a special blessing) would be lost if the word rock were omitted. Also it is needed for the reference to the pasuk