Why did Moshe have to leave the town to Daven?
Rashi and Ramban #2 (citing the Mechilta): Because the town was full of idols. 1
Ramban #1: Because, whereas he generally Davened in his home, this time, to counter the affects of the plague, he wanted to raise his hands heavenwards for the thunder and hail to stop, and it would not have been appropriate to do so in town.
Riva: This Makah was in the field. Moshe wanted to see the damage to the trees, and then Daven for it to cease].
Oznayim la'Torah (citing the Ma'asei Hashem): Because since the plague took place in the field, it was appropriate to Daven there.
Sifsei Chachamim: Because the Egyptians' idols (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 to Daven. 2
Why did Moshe find it necessary to inform Par'oh that he would Daven when he left the precincts of the town?
Ramban: Because, as opposed to the previous occasions, when Par'oh asked for the plague to be removed only the next day, Moshe did not need to inform Pa'roh where he would Daven, this time he asked for the hail to cease immediately, and Moshe was obligated to tell him that he would only Daven as soon as he left the precincts of the town. 1
Sifsei Chachamim: Refer to 9:29:1:5.
Riva: Refer to 9:29:1:3.
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 (miraculously) cease simultaneously - from one moment to the next. 1
Seforno: To emphasize the fact that the cessation of the plague was not because Hashem thought that Par'oh had done Teshuvah - as the next Pasuk clarifies.