What can you use "gold bars" for if you have all 5 styles for all buildings and estates?

You can purchase the 6th style, the "gold facade" option which then gives you all of the previous styles with the ability to change which appearance you use (without changing your flowers) AND it also gives you additional flowers. The cost varies for each building and until you have enough gold to get the "gold facade" you won't increase your flowers, but you can purchase previous versions of each building individually. Hope that makes some sense.