On Shimano Ultegra and Dura-ace brake calipers there are round-head slot screws in the lower section of the brake pad holder. In the BR-6600 spare parts catalogues they are labeled as 'tire guide' (item 13), however they are not mentioned in the user manual. They are also absent from Shimano 105 brakes and from comparable SRAM brakes. What is their function?
The 'tire guide' screws are for deep section carbon fibre rims.
The idea of the brake shoe holder is that you replace just the rubber bit, not the entire shoe. In this way you do not lose your settings, as happens when you replace the entire brake block, not just the rubber bit.
For speedy wheel swapping, i.e. in race conditions, the 'tire guide' exists. These have been round for a few decades and were originally conceived with square, box section rims in mind, e.g. original 'Mavic' rims such as the MA40.
The problem with deep section rims and tire guides is that, when the brake wears down, the rim can come in contact with the tire guide. The last thing the rim needs is to be scored all the way round by the metal tire guide. This could damage the rim beyond its cosmetic appearance. As for the brake block, the last thing it needs is for the tire guide part of it to be damaged by being worn off onto the rim. This would not be a problem if the whole brake block was swapped out when worn, but, nowadays, it is just the rubber bit that gets swapped out.
Enter the 'tire guide screws'. These are made of a plastic that wears away quicker than metal does. So, when the brakes are worn, the plastic shoe hits the deep section of the aero rim instead of the metal tire guide. The rider notices this through what the brakes feel and sound like, to hopefully cotton on that it is time to put some new brake block inserts in. Instead of the brake block holders and the rim getting damaged, these simple plastic screws get worn down. Replacement plastic screws don't come with the R55C carbon rim brake pad inserts, although they do come with the replacement brake blocks:
Personally I think it is interesting how patents like this work. There is 'prior art' with tire guides covered in a plastic coating from the pre-deep rim era, Shimano update the concept with nothing more than a plastic screw and get a patent out of it.