Rashi points out that uniqueness would lead to misconceptions as to the meaning and purpose of the universe as well as a fundamental misconception of Hashem
Bereishis 2:18
And the Lord God said, "It is not good that man is alone; I shall make
him a helpmate opposite him."
Rashi
It is not good, etc.: [Gen. Rabbah] Lest they [people] say, “There are
two dominions: the Holy One, blessed be He, is alone among the
heavenly beings, and He has no mate, and this one [man] among the
earthly creatures, has no mate.”
Rav Hirsch shows the meaning as
It does not say: לא טוב לאדם היותולבדו, that it is not good for man to
be alone, but לא טוב היות האדם לבדו "this is not good, Man being
alone"
that is:
as long as Man stands alone it is altogether not good, the goal of
perfection, which the world is to attain through him will never be
reached as long as he stands alone. The completion of "good" was not
Man but Woman, and it was only brought to mankind and the world by
Woman.
...
A task which is too great for one person must be divided, and just for
the accomplishment of the whole of Man's mission, Hashem created Woman
for Man.
Rabbi Levi translates the term עזר כנגדו into English as
a help meet unto him
which means a help which is appropriate to him
He adds in explaining the idea of עזר כנגדו that
In a similar way, the wife is to take over a part of the obligations
which comprise the great task of mankind, and thus make it possible
for her husband to accomplish more perfectly the part that is left to
him. But this is only possible if the being who is to be an עזר to him
is כנגדו not עמו, does not work at what he is doing, but works
opposite him. Were the other being a man, he himself would have all
the obligations on his shoulders, and he would again need "help". That
is why it is אשה, a "feminine" man, who stands, not with him, but
next to him, נגדו, who works at another point in the same line, so that
each one of them fills a separate position, and they mutually
complement each other.