John declares that "there was a rainbow
around the throne".
It means that
though the
throne is one of
judgment, it is
not a throne to
bring the
world to an end.
God is keeping
in mind
His covenant with Noah. The rainbow is
a token of God's covenant with all flesh,
never to destroy the inhabitants of the
world like in the Flood. (Genesis 9:11,17)

As judgment falls on the earth, fears
might arise that God is about the destroy
the world.

The appearance of the rainbow shows
that God will keep His promise and
covenant. The earth is to be judged, but
not to come to an end.