Wozu die Medaillen gut sind kann ich auch nicht sagen, aber wo du sie findest schon...wenn du ins Menü "Gegenstände" gehst, dann gibts da glaub ich ganz unten die Monsterfibel und unter anderem auch eine Abenteuerchronik, wo die Anzahl der gefunden Medaillen verzeichnet ist.
*Edit* hab was Interessantes gefunden:
Medals are gotten almost everywhere. Usually you can find 1 or 2 medals in random stuff(vases, plants, desks, rocks, gadgets, machines, etc), sometimes you will find up to around 10 from random stuff, and chests also contain medals. From 1 to 30+. It'll take a while before you can first spend medals, but you can buy Hearts and Stat potions with the medals.
Hearts are like SwordMaster heart, Assassin Heart, Barrier magic heart, etc. These will raise the class level of that shadow(Character) by one. The potions range from Agility(Only one really worth buying) to hp and attack/defence potions. These will raise a stat by 1, or HP/MP by 3, there are also Super HP/MP potions to raise it by 10(Cannot be bought, only found)
Each item costs 2 medals at first. Once you buy one, the next one will cost 2 more medals. So if you buy 4 monk hearts, and 3 agility potions you have spend 32 medals, but the next monk heart will cost 10 and the next agility potion costs 8 more medals. This can quickly drain your medals if you only buy the same stuff, but I cannot recommend using medals on normal stats except Agility, as you will find plenty of these potions and a heart will accomplish alot more than +1 stat.
Just keep looking, check everything which even comes close to something you have searched before. Search fountains, random rocks in your way, almost everything, even ventilators in machine buildings, or random pillars will be searchable.