Currency backed by gold, not an illusion.
what can you do with gold that you cannot do with bitcoins?
it has value because is rare, period
can be used in manufacturing of some electronic components, but it is not only option... ~90% (?) of its value come from limited supply not from any kind of actual usefulness
same with diamonds
Gold does not have value "because it is rare, period." There are many rare things - for example a blue cactus is rare, why has it not become the foundation of the global monetary system?