Perfectly round holes on new Schwalbe Pro One tire
It is an increasingly common practice for manufacturers to add tire wear indicators, which are little divots like the ones on your tire. Otherwise, there is no objective measure of when to change a tire apart from your mileage log (and riders of different weights should be expected to wear out tires at different rates). Basically, once you can no longer see the holes, you should toss the tire.
Perhaps you already know this, but you can then rotate your front tire to the rear. One should not rotate tires as we do in cars, because our rear tires wear a lot faster. I always knew this in theory, but seeing this happen with the wear indicators really cemented the concept. In any case, you want the freshest rubber on the front for control. However, rotating a very lightly worn front tire to your rear wheel is perfectly fine.
In principle, this type of wear indicator could also be useful on rim brake rims. I have not seen them as often, however. The Pacenti SL23 rim definitely had them, but that was a bit of a niche rim. Otherwise, one has to estimate rim wear via calipers or by seeing if the brake track has become concave. The increasing adoption of disc brakes may mean that manufacturers won’t bother with these indicators, because disc rims don’t take wear per se.
Just in theory: what happens if you ride the tire after the wear indicators are gone? Remember that tires have a rubber tread, which is glued to a carcass made of rubber, cotton, or perhaps other materials I’m not aware of. (Illustration at Schwalbe tires.) If you wore off the tread entirely, you would now have either the puncture protection layer (if present; some racing tires may forego one) or the carcass itself in contact with the road. I am not an engineer. However, the carcass materials were designed to hold a shape, and not to contact tarmac. I would expect grip to go down substantially, and I would not expect the carcass to be durable in that manner. Once you wore out the carcass, you would presumably tear a hole in the tube. In any case, I don’t know how much tread is left once the holes are worn down. It could be possible to safely ride your rear tire once the wear indicators are gone, but not for long. Under no circumstances would I do this on the front tire.
Come to think of it, on the forums I used to frequent, I would sometimes hear about riders wearing their tires down to the threads. That was a reference to the threads of the tire carcass. Many higher end clinchers and tubulars in the 2000s had cotton carcasses, which could be very supple. Presumably (NB, I’m not an engineer) this was supposed to reduce energy loss due to hysteresis, which is a primary driver of tire rolling resistance.
Edit: Some time after I wrote the answer above, @A.Jahin asked a question where they posted a photo of a tire where the tread had been completely worn out in one spot. This, below, is an example of wearing the rubber tread out completely. You can see that the threads of the carcass have been worn.