Why does the Torah refer to Dan as "Me'asef le'Chol ha'Machanos"?
Rashi (citing the Yerushalmi in Eruvin, 5:1): Since Dan was a numerous tribe 1 , they traveled in the rear, and if someone dropped something, they would return it to him.
Rashi #2: To teach is that the tribes traveled (one behind the other) like a beam 2 - and Dan was last.
Rashi #3 (in Yeshayah, 2:12), Da'as Zekenim and Hadar Zekenim (on Pasuk 2): Those who travel in the rear are called 'Me'asef' because they pick up all the stragglers. 3
Rashi: This follows the opinion that they traveled like a box (in which case they needed to be numerous in order to be able to spread out to the width of Machaneh Yisrael). Refer to 2:17:2;1.
Refer to 10:25:151:1.
See Oznayim la'Torah, who elaborates.
QUESTIONS ON RASHI
Rashi writes that Dan was [the most] numerous qtribe. But Yehudah was bigger?
Moshav Zekenim: Dan was bigger than the Sheivet (Chizkuni ? Degel 1 ) in front of them.
Mizrachi: Yehudah was first (Gur Aryeh ? because he was king). Dan was the biggest of the other Shevatim,
Emes le'Yakov: It is clear from Yehoshua 6:9 that "Me'asef" refers to Sheivet Dan, and not the Machaneh.