What people did back in the day when there were no currency? They traded goods for goods. It's all about demand and necessity. If you need 2 potatoes, but the only option you have is to trade 3/4 onions. And that's the only option you have in this situation. So you will be willing to trade that for the 2 potatoes. Because you need it.
So if you compare that potatoes and onions with Bitcoin and goods, then you can easily value it without the help of USD. It's just easy to do it if you compare the value of BTC with USD. That's why people do it. We just need a new system and calculation for every item with BTC's value, and then you can get the actual value of BTC.

I may have explained in a complicated way. But it is what it is. Demand is the key for value.