T7 gear 10 / 25 man

Okay, from my understanding, you can buy 2 pieces (gloves and chest) of 10 man T7 gear with emblems of herisom which you obtain from either heroics or 10 man raids. Then, you can buy 2 pieces of (legs and shoulders) of 10 man T7 gear with emblems of valor which you obtain from 25 man raids. There is also a 25 man T7 set. Which you obtain by doing 25 man raids.

My question is; why would you obtain 10 man T7 gear from 25 man raids? Was there a point that Blizzard is trying to make here by making the pieces of the 10 man T7 set purchasble by only 25 man raids?

I understand that you can do a 10 man raid and hope for a drop of the pieces you couldn't purchase. But why would a person want to buy 10 man T7 gear with 25 man emblems?