Well, it's something that can make or break a deal.
Imagine you want to buy a weapon to fight people using evil druid, so you want a weapon that gives damage against undead, right?
Well, you go to the shopping board and see an amazing weapon, 60 atk, 25% damage on undead, and it's 10m.
But... is it undead property or undead race?
Evil druid makes you turn into undead property, but what if the 25% damage is against the race undead?