Probably since it’s their equivalent of the latin e? In my understanding the Russian cyrillic e is pronounced “ye”, so э is closer to the latin e.
Note: I don’t speak Russian so anyone with more knowledge feel free to correct me.
Also: This is for Russian cyrillic, I believe in Ukrainian cyrillic for instance, it’s the other way around, so e is e and э is ye.
Oh yes, it’s the legendary kurkkumopo! If you know, you know.