Today's systems have a centralized source of truth: corporate databases. There was no other way to build applications that involve multiple users interacting with each other, so companies built those systems and profited from providing a solution.
With Ethereum, we can write simple programs that run on thousands of computers and come to a consensus on the result. Nasdaq doesn't need to tell us the result of a trade or who owns what. We can build equivalent systems that no one controls. The result is a transfer of profits from middlemen to the rest of humanity.
Finance is easy to think about in a decentralized manner since it's all information that computers control. But these systems can apply to real world interactions as well by building reputation systems on top of this global computer. If you don't abide by the rules of the decentralized system you're participating in, your reputation score will go down and people will stop interacting with you.
In this manner, Ethereum can be used to replace every marketplace startup. Uber and Airbnb are so successful because they benefit from strong network effects. Decentralized applications also benefit from strong network effects: every new decentralized Uber application will use the data from the ones that already exist to bootstrap themselves. The user base will never splinter like it does when new centralized competitors attempt to enter the market.
Ethereum gives mankind the ability to coordinate interactions between ourselves more powerfully than ever before. Social systems that once seemed too unwieldy or too anarchic to work out might now be feasible, and they could make us wealthier and more free.