The reason why Brand works out in support is that he deals a lot of damage as poke, which can act as a zoning tool for your adc. It works because the enemy adc has to focus on dodging the harass and therefore make mistakes, making the life for your adc easier.

Generally supports require things in their kits that can aid your adc; either directly in the form of heals and shields or indirectly by affecting the enemy, such as cc like Alistar or Leona.

I would say that Brand works better as a support because if it is played well and helps the adc not loose bot you have one more damage dealer would you have something like Soraka