Thats a valid question @pandorasbox. If you compare it to any other currency the value always mainly comes from the trust that people give that particular currency. Why do people trust the USD? Why do people trust Bitcoin Why do people trust gold?
So the trust can be based on many factors scarcity, functionality, brainwash, comfort, security, immutability etc. The trust in Iota comes from all the points i just mentioned in the list above. Main use is of course being the currency of the network aswell as the currency for everything that will run through the network