If you think every dollar in existence has it's share of gold in Fort Knox (or anywhere else for that matter), you're in for a shock. The dollar hasn't been backed by gold since the 1970's. What it is effectively backed by is the future economic output of the US - essentially the future labour of the people of America.
But again, if you calculate the number of dollars in existence and figure out how many each US citizen would have to back.... the numbers are just as bad as the gold.
The reality? The dollar is backed by nothing more than confidence. That's exactly the same thing backing bitcoin. The only difference is the dollar is more widespread, more entrenched in the system, and has more confidence. As for real value though? There's not much in it.
Yes people give value to bitcoin same as we give value to USD, EUR , CNY and other currencies. The main difference is that nobody can generate more bitcoins as they have fixed cap amount. But when you look at ethereum for instance it has unlimited cap, they haven't put any. For the value to be stable when and if we get mass adoption we need fixed amount as we don't want the value of the currency that we use to decrese over time (which is exactly what is happening to fiat currencies as goverments tend to print out more money).
The main purpose of bitcoin is to be used as normal currency and most likley it is going into direction to be used as online currency. The other thing is that people see it as an oportunity to profit from it. Thats why most of alts came to life to try to profit from them. And because we are still in early phases bitcoin don't have a stable price to be used more as I am sure if the price was a bit more stable we would see more merchants accepting it.