I haven't seen this part before. What is it? What is it used for?
This part is 3386 Plate 1 x 1 x 2/3 Half Round with Side Stud.
I recently encountered this part for the first time and wanted to share more about it as it is new to me and looks very useful. It was launched in 2023 and appeared originally in 3 speed champions sets:
It's even a bit like the classic Erling brick when the brick is used on it's side:
This part shares its footprint with 24246 Tile Round 1 x 1 Half Circle:
It does have some unique advantages. It's very easy to sneak this part in where it would otherwise be difficult to create a studs out connection. Here are some examples from the first wave of sets:
Another advantage of these parts is that they can be used to recreate the 5 plate tall bricks with studs on side in a way that should be usable in most contexts where these elements are needed:
The geometry of these parts also makes them useful for microscale building. Here's a quick example of a train car that uses these elements to attach the wheels:
The rounded back of the part also allows it to be used at varying angles as demonstrated in this quick X-wing design:
This looks like a part that could open up a number of new building opportunities.
Here's a final clever example of this part being used upside down to create a tiny toilet on the Concorde: