What is the significance of the fact that the flax and the barley were smitten by the hail, whereas the wheat and the spelt were not?
Rashi: The hail broke what was hard and brittle, but not what was still soft and supple. 1
Ramban #1 (citing R. Sa'adya Ga'on): Moshe was telling Pharaoh that when he prayed for the plague to cease, he would not be able to save the flax and the barley. Only the wheat and spelt could be saved. 4
Ramban: It was a warning to Pharaoh that if, he did not relent, Hashem would send the plague of locusts, which would finish what the hail had left over.
Rashbam: Having said that the hail destroyed everything in the field, the Torah needed to inform us what the locusts still found to eat. 5
Seforno: It is to demonstrate the extent of Pharaoh's wickedness - even though he saw that Moshe's prayers had prevented the hail from destroying everything, he hardened his heart and stubbornly refused to give in.
Because it grew only later ("Afilos" means late). This also served as a lesson to Pharaoh - that someone who stands up to the storm will perish, and it is one who bends before it who will survive.
Tosfos ha'Shalem (6, citing Moshav Zekenim): Why was it a miracle? The flax and the barley were smitten only because they were ripe! Indeed, the hail would not break it; the miracle was that it was not stricken with Shidafon. The letters of Afeilos spell Pela'os.
To give Pharaoh a chance to do Teshuvah and save the land from total devastation (Refer to 9:16:2:1*).
See the Ramban's objection to this explanation.
Rashbam: Hail tends to destroy whatever is hard, whereas locusts consume whatever is soft.
Rashi connotes that what was still soft and supple was not stricken. Why did the Torah record (in Pasuk 25) that the grass of the field was smitten?
Rashi: Pasuk 25 refers to vegetation that was fully-grown and brittle.