Personally, I haven't seen this before but I can take an educated guess and would say people do it to help improve the saturation of the dry coffee bed when pouring the bloom.
There's lots of techniques that coffee makers employ when pouring hand brews to deal with the issue that often when you're pouring the bloom, not all of the coffee grounds are getting wet. This means that you can get an uneven extraction, this is why you see a lot of pour over recipes asking that when pouring you pour from the center outwards in a spiral to make sure you saturate all the grounds evenly.
It would be a lot easier to evenly saturate all the grounds if the coffee wasn't in a conical shaped brewer as the surface area isn't flat, this means there is more coffee to saturate in the middle than at the sides.
Essentially the reason people would create this shape on the top of the coffee bed, would be to obtain a more even extraction by creating channels in the top of the coffee bed for the water to run through.
Personally I think this would achieve a marginal difference in creating a better extraction and is purely an aesthetic achievement this brewer has created or seen and recreated. You can achieve a better extraction by pouring better.
