Knob underneath oem led headlight

Just curious, what is this knob underneath the headlight. I thought it was a drain or simply a cap so I can get rid of the moisture inside, but seems more like an adjuster than a cap.

 

And as soon as I created this post, i get a different grip on said uncertain cap, and it pops right off. Rubber ring keeps it nice and snug, so it wasn't screwing off as I expected it to. That said, water just dumped out as soon as I removed it, so it worked as intended. Now to figure out why water got in.

 

It will never work again, you let the headlight fluid out.

 

Literally changing out the blinker fluid haha

 

Oldie but goodie, blinker fluid and bucket of steam:

 

Moisture can accumulate via ambient humidity. These lights are not air/vapor proof, hence the drain plug. Put the plug back in and carry on.