How significant are 70 date-palms for a nation of millions?
Targum Yonasan: The twelve springs of water corresponded to the twelve tribes and the seventy date-palms, to the seventy Zekeinim. 1 It is fair to assume that they held sufficient water and dates to supply all the people),
Elef ha'Magen: Refer to 33:8:151:1. They received some Mitzvos at Marah; this was like a mini- Shavu'os (Matan Torah). Here was like a mini-Sukos (Avodah). They were taught Mitzvas Arba'ah Minim (they are taken all seven days in the Mikdash; we bless on the Lulav, from date trees) and Nisuch ha'Mayim (the springs).
To demonstrate the esteem in which Hashem held the twelve tribes and the seventy elders.