Secondly it will limit the growth of future enemies, example is Rome with its many Hellenic neighbours which would consider giving you tribute in exhange for protection.
If you look towards the Balkan region you will see on the culture/religious mapmode lots of Hellenic provinces under tribe leadership. With
an improve relation, and alliance and (if you're in a rush) a minor gold payment (10-25). You will gain above 100 relation and able to turn them into a Tributary or Tribal Vassal , when this is done the relation slot you used allying them will be free to be used on the next tribe. This strategy can be used by any large nation and can be used on most tribal and smaller nations. Giving you the option of divide and conquer large parts of the world before you even begin military conquests in these regions.