Hello Max,
you're right: all the parasitics are available after extraction. However, a SPICE simulation
1. relieves you from the task of adding several of them, especially if they are distributed over several metal layers - and SPICE won't forget any of them ;-)
2. if such a node is connected to a device terminal, there will probably be voltage-dependent caps adding to Ctotal. SPICE can add those voltage-dependent caps, which might be desirable if you wish its Ctotal in a real application.