At one end of the wire is a 3.5 mm Jack plug. On the other end of the wire is presumably a glass LED.
When connected to the phone, nothing happens. The LED does not start to glow.
Also, this wire doesn’t work as a microphone. It’s also not an earpiece.
What is it for?
Yes but nowadays you’d buy a USB version since few phones have headphone jacks. They’re on Amazon.
Here’s an old style headphone jack ir blaster adapter:
https://www.ebay.com/itm/404693924792