Rashi: Because "He'alos he'Anan" means 'the Cloud departed', whereas 'Alos' would have meant 'the Cloud rose'.

Seforno: It means that when the Cloud lifted, Yisrasel would travel in the direction that the Cloud indicated.

Oznayim la'Torah: The Torah writes "ve'Acharei-Chein Yis'u" and not 've'Achar-Chein', because 've'Achar-chein' implies that they traveled immediately, whereas in fact, they only traveled after the Kohaniam had blown the trumpets and Moshe had given the order to travel. ¹

Ramban and Seforno: In praise of Yisrael, who followed instructions without complaining, even when the conditions were unfavorable. Sometimes they encamped in a location that was empty and where hyenas howled; ¹ Sometimes they would remain in such a place for a long period of time without searching for better camping -ground. ² Sometimes the location was ideal for them both personally and as regards grazing for their cattle; ³ nevertheless when the Cloud moved on they did not complain; ⁴ Sometimes they stayed in a location for just one night, leaving them insufficient time even to unpack ⁵ before they moved on; ⁶ and sometimes they had barely managed to get organized when they broke camp, or they remained a long time in one location, and just as they had begun to settle down, they suddenly had to move. ⁷

Oznayim la'Torah: Sometimes the Cloud remained in one location for a long period of time which was so unpleasant that a day seemed like a year - yet, when the Cloud remained, they followed suite. ⁸ Sometimes, the Cloud remained in a pleasant location for what "seemed like a few days" ⁹ - yet when the Cloud traveled, they broke camp and traveled too. ¹⁰ Sometimes, it was only overnight, leaving them barely enough time to rest from the previous journeying - yet when the Cloud traveled, they broke camp and traveled too. ¹¹ Sometimes the Cloud remained only a day and a night and rose before daybreak ¹² - yet they rose in the night to be ready to leave at the crack of dawn. ¹³