Building Licence
The +10 Building Licence is probably the single most useful item in the Store compared to Gems spent on it.
Every player can use 10 more buildings on their island, especially when residences and Farms become necessary to have in bulk at the higher levels to maintain an army and Horse/Bread Production.
The Licence is not giftable to friend players to avoid it being abused by multiple account users.
Purchase the Licence with Gems and find it placed in your Star Menu. Click on it and place it on your Mayors House to gain the +10 Licences.