The originator of the currency often has a purpose they wish to fulfill, e.g. do Artificial Intelligence research, provide co-working spaces across the globe, or create a credit card powered by Bitcoin, and they explain this in their White Paper.

Some believe a cryptocurrency has to have some purpose if the cryptocurrency is to have value; while others believe that all you need is a ton of users (coin owners) who all just simply believe the cryptocurrency has value.

When you look at the old-style fiat currency (like dollar bills or Pounds Sterling) most people give those pieces of paper some value by just simply believing it is worth some value - its a community agreement to accept & use such pieces of paper in exchange for goods and services.