Gold and diamonds have industrial uses and so some inherent value (even if a seller's cartel in both cases overinflates them). Fiat currencies are backed by big militaries, which, again, is an inherent value.
Sure, you need to get money into bitcoin and then out in order to take advantage of that property, but the ability to do that is getting easier and cheaper all the time.
Bitcoin, Litecoin - money transfer
IOTA - micro transactions without fee ( Internet of Things )
Ethereum - contracts
Monero - private, non-traceable transactions
the list is long. 99% percent of these projects will fail, but 1% will create added value to the world.
Make no mistake though, bitcoin is already valuable as a value store, especially for wealthy individuals who live in countries not quite as stable as the US. It also has a place as a convenient high risk investment at the high risk end of any investors portfolio who is looking to increase their risk adjusted return.