My instructions for kneading strudel dough include slamming the dough against the floured board a few times during kneading. What is the purpose of slamming the dough?
I suspect that there isn't a functional reason per se. It's just a description of what the kneading motion becomes when you have a lot of dough in your hands.
If you're making a small batch, then you can just fold the dough repeatedly. But if you're working with dough made with 2-3 kg of flour, novice bakers are prone to taking it easy, leaving the dough lying on the table and kinda massaging the sides a bit. This won't knead the dough properly.
The real kneading of a large batch of dough requires the appropriate large movement. You have to grab enough of dough with your right hand, stretch it sufficiently, and fold it over the half you're supporting with your left hand. You have to repeat this several hundred times before it's done, so it's best to do each individual fold quickly. You're kinda throwing the right edge of the dough against the table, to the left of the left edge. The combination of picking up enough dough on the right side, stretching it far enough before folding, and doing this with speed, is what produces a resounding smack at the end of the folding motion.
When you knead a small batch, say 500 or 750 g of flour, the motion naturally transforms. Now you're more pushing the dough to the right with the ball of your right hand. No slamming occurs.
There is also a kneading technique where you heave the whole dough ball in the air, slam it and do a kneading movement from both sides. You can see it here: https://www.youtube.com/watch?v=cbBO4XyL3iM&t=185s.
Again, I don't think that the slamming does something beneficial to the dough. Try imagining doing this motion, while you're moving your hands quickly and energetically, and at the same time ensuring that the dough has a soft and gentle landing on the table. It would be quite awkward. Instead, what you do is to move your hands first up, then down, then fold from the sides. The downward motion of your hands, especially when combined with the drooping of a heavy batch of very soft dough (not visible in the video, as the batch is small) would automatically produce a slam. So this is what the motion feels like to the baker, and that's how it gets described and taught.