The ability to set an offspec loot preference was originally to get DPS players to tank and heal for 5-man pugs. These are people who normally play DPS, but Blizzard wanted to draft them to fill spots filling up groups. People said I don’t want to do that because then it rolls tank or healing gear I don’t want.
Blizzard’s “answer” was the offspec loot function. Why it goes the other way… that’s a good question. Probably to fill up raids with DPS, since there’s a glut of tanks in raids? Or more likely Blizzard just didn’t put much thought into it.