It's just because they want you to learn that there is an "oda" in the sentence which is pretty important. If you said elköltöznek, ideköltöznek, visszaköltöznek, etc etc.. it would have a different meaning. If you wanna eliminate the "there" you will have to say "elköltözni" like "A lányok elköltöznek a folyóhoz." - The girls move to the river. There you go, now you have no there. xdd
That makes sense, thanks. I do understand that the preverb has an important function in Hungarian, although I've never been sure exactly what it does. And I don't know how to convey that meaning in English. "The girls move there to the river" sounds like something you might hear in a song lyric, but it's weird in everyday speech.
In my opinion the oda preverb is not so important in this case beacuse 'költöznek' implying itself a direction (a lányok a folyóhoz költöznek means the same). It is more important, for example, in the case of megy-Megyek a folyóhoz and odamegyek a folyóhoz means pretty different things