Why does it discuss where springs go?
Radak: Also this Hashem did to water for the sake of His creations. When He gathered the water, He left some in the land, and made them salty collections (lakes). Since they do not flow, they would spoil and ruin the air. Therefore, He made them salty, to fix the air and prevent spoilage. He left some sweet (non-salty) water in the dry land, and they are swallowed in the land. He sent from them in a small number of places to flow for the need of the creations to drink, to irrigate crops, to cool the land, to give moisture to living beings and to moisten the dry air that comes from the salty sea-water. Even those in Midbaros are useful, for people traversing Midbaros and for living beings there to drink.
Malbim: It shows how He arranged everything for the need of the creations.
Why does it say "ba'Nechalim Bein Harim Yehalechun"?
Radak: He did not send springs over the entire land, lest they damage vegetation and land-dwellers. He sent them to spread in valleys between mountains, called Nechalim.
Malbim: Water vapor rises, and [rain] falls on mountains. It comes out of the ground and springs arise, and they descend to rivers between mountains. Hashem sends springs to rivers that go between mountains.
