They are known as spigot rings. They make sure that the wheel sits centered on the hub, where the centre bore of the wheel is larger than the hub of the car.
If you have factory fiat wheels you should not need a spigot ring to fit them, as they are made to fit the car.
If you've got aftermarket wheels then you may well need spigot rings.
The hub on the Stilo is 58.1mm; so you'll need a spigot ring to go from 58.1mm to whatever the centre bore of your wheel is.
For example, my 18" OZ wheels have a centre bore of 60.1mm, so I have 58.1mm to 60.1mm spigot rings.