Why did Moshe have to leave the town to pray?
Then why did he not do so on the previous occasions that he prayed?
Ramban: Because, as opposed to the previous occasions, when Pharaoh asked for the plague to be removed only the next day, Moshe did not need to inform Pharaoh where he would pray, this time he asked for the hail to cease immediately, and Moshe was obligated to tell him that he would pray as soon as he left the precincts of the town. 1
Sifsei Chachamim 2 : Because their idols (alias the sheep, which they worshipped) were generally kept in the fields, and it was only now, when the G-d-fearing among them gathered the sheep into their houses, that Moshe needed to leave the town.
What is the significance of Hashem's words "Lema'an Teida ki la'Hashem ha'Aretz!"?
Ramban: Refer to 8:18:1:1**.
Seforno: He said this with reference to the fact that, thunder and hail would (unnaturally) cease simultaneously - from one moment to the next. 1
To stress that the cessation of the plague was not because Hashem thought that Pharaoh had done Teshuvah (as the next Pasuk clarifies [Seforno]).