Elastomer in bicycle trailer hitch: is it important?
A spring, of course, is just a piece of wire.
When you bend a spring, you're just bending a piece of wire.
In a perfect world, the wire would bend evenly along it's entire (coiled) length.
In the real world, particularly during sharp or extreme bends it's possible for the wire to kink.


Either way, the kink becomes a weak spot and as with all metals when you flex them repeatedly in one place, they will break.
The flexible insert allows the spring to bend a small amount on it's own before the insert comes into contact with the spring's interior. That has the effect of spreading the bending forces across more of the spring's coils ensuring the spring bends more evenly along it's length and preventing a kink.
You could try replacing the broken polymer with a length of stiff hose filled with expanding foam. It might be weaker than the original, depending on the stiffness of the hose and foam, but they're common materials and easily replaced.
Just keep in mind: you or someone else, turned the bike so sharply you snapped the original material. Try to avoid doing it again.