In HeartGold I mean. I got all 28 in Platinum and don't remember getting anything.
In Heartgold/Soulsilver, there is little clarity about what does happen, but you must go to the arceologist station to get the item/s
Well, in pokemon pearl or daimond you go to the little boy near the ruins and show him an Unkown then he will give you a Alphabet seal depending on the unkown
i find them usefull in wifi to spell my name or somthing I like.
You also gain acess to hidden cave on the route south of veilstone city. there used to be a guy there in front of a wall but when you go back with all the unknown there will be a path and a rare candy awaits you