Of what importance is it to know that Ashur left that land to go and build various cities?
Rashi and Seforno: It was because Ashur saw them begin to build the Tower in rebellion against HaSh-m, that he fled in order to avoid joining the rebellion. 1
Targum Yonasan: The Pasuk does not mean that Ashur left that land, but that Nimrod left it (for the same reason that Ashur left it according to Rashi). 2
Ramban: The Pasuk means that Nimrod went to Ashur, to capture Ashur 3 and rule over it.
Perhaps in that merit, HaSh-m sent Yonah to save Nineveh (the capital of Ashur) from destruction (and they did repent). The very next Pasuk refers to the building of Nineveh and calls it "'the great city." (Even so, Ashur joined the nations to fight Yisrael