It gives a few Gentiles the opportunity to be alive for the period known as the 1,000 year reign. They are a 'remnant' of all gentiles who have have been alive while the 12 Tribes are whole as their resurrection includes the sons of Abraham. All of Zec:14 is about how the 1,000 year reign starts and goes and the 'remnant' is the ration given in the last few verses of the previous chapter as that is where the passage starts.
And it shall come to pass,
that in all the land,
saith the LORD,
two parts therein shall be cut off and die;
but the third shall be left therein.
And I will bring the third part through the fire,
and will refine them as silver is refined,
and will try them as gold is tried:
they shall call on my name,
and I will hear them:
I will say,
It is my people:
and they shall say,
The LORD is my God.
During those 1,000 'feasts of the tabernacles' they attend in that passage the 12 Tribes learn how to be Priests in a sinless kingdom and when the Church is wandering around the planet between learning how be Kings and that would include building cities and showing 'the flock' how to make pastures using voice commands and that actually means the Holy Spirit will do it for whomever speaks the words.
In the New earth era that follows Judgment Day both would reside inside of Jerusalem and they would use all they leaned to help the ones that will be living outside of the walls of New Jerusalem. The relationship of shepherd/flock is the flock is the larger group by far. It starts out that way and only the ones outside the walls will have children, 100 of them to be exact.
Pets made up of the flesh that Adam was given dominion over will be alive again so coming in 2nd is not all bad news. Being a fallen angels only holds bad news for their eternity.