They do serve a purpose, but €37 seems really overpriced IMO. I have a 8x3 that I got off of eBay for $35. I'm no expert on honing, but using the Norton baseline, I'd guess 4-6k depending on the slurry. I have a beater razor that I am learning to hone with and I did manage to get it shave ready off that stone. I set the bevel on a DMT 1200 and proceeded to hone on slurry using ham-fisted dilutions until pure water. I finished on lather, maybe 50-75 laps. I've tried to recreate the success with other razors but this stone seems to only like vintage Solingan steel. These days it mostly acts as a base for lapping film.
Member of the illustrious Order of Pinaud and battle hardened Boar Army.