Glory liked pretty things.
Well we all knew that but she made it seem like it was crucial to the ritual that Dawn wore that outfit?
She did make it seem that way, but I highly doubt it was crucial, all it took after all was blood. It is most likely was just a symbolic part of the ritual. Also, before they knew that Dawn was human, Glory expected that it might be any object, but also this might have been completely false. It is possibly that the key had to be transformed into human, it might not have been possible to transform it into a bike or something, we just don't know that for sure.
Glory liked pretty things.