What was the significance of the twelve springs and seventy date-palms?
Ramban (citing Ibn Ezra): The Pasuk mentions it to stress the contrast between it and the previous encampment (in Marah) where the water was bitter. 3 Date-palms do not flourish where the water is bitter.
Seforno: Even though the location was so convenient, they did not hesitate to move on when ordered to travel. 4
One for each tribe (Ramban and Targum Yonasan). Toras Avigdor (Beshalach p.2) - Hashem wanted each tribe to be by itself. We similarly find (24:4) that Moshe built 12 Matzevos, one for each tribe.
Ramban: The seventy elders sat in the shade of the date-palms - all this in praise of Hashem, who miraculously prepared it for them from the day of the creation in an arid land.
In fact it was because of the pleasant conditions that they remained there longer than most of the other encampments in the desert - as hinted in the words "Vayachanu Sham" (Ramban).