Moving data about within a blockchain is trustless - you don’t need to trust the network to move money. You can set up smart contracts to move money under certain conditions (usually other transactions)

When you want to interface with the real world, say trigger a smart contract on a stock price move - who do you trust to get that data? You’ve reintroduced a trusted authority.

This is the oracle problem. Now, if like me you don’t give a crap and are happy to trust middlemen in your day to day life because I trust banks over random merchants a million times… big whoop. But if you’re a cryptocurrency fetishist it becomes a big deal.