The assimilated forms are found only in the artificial language of Homer, Hesiod, and their imitators, and nowhere in the living speech. They are commonly explained as derived from the contracted forms by a process of ‘distraction,’ and as inserted in the text for the sake of the metre. Thus ὁρᾷς,
βοῶντες, the spoken forms which had taken the place of original ὁράεις, βοάοντες, in the text, were expanded into ὁράᾳς, βοόωντες, by repetition of the α and ο. While the restoration of the original uncontracted forms is generally possible, and is adopted in several modern editions, a phonetic origin of many of the forms in question is still sought by some scholars who regard ὁρόω as an intermediate stage between ὁράω and ὁρῶ. It will be observed, however, that the forms in 648 can be derived only from the unassimilated forms.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].