Quantum simulators will eventually allow scientists to develop new pharmaceuticals and materials on a computer, rather than laboriously manufacturing them and then testing their properties in the lab or on living organisms. Currently this is done on computers with a limited success, and uses up many years of time on super-computers. A quantum simulator will be many times faster. It will be more accurate, as nature is quantum so our model of it should be as well.
Universal quantum computers can do other things like search large databases (e.g. find pictures of Aunt Maude in your photo collection) or help business save money (work out how to deliver parcels in the most cost-effective way, something an ordinary computer struggles to do).
One of the first "killer apps" was an algorithm that would run on a quantum computer and allow the reading of secure messages on the Internet. However, this is only one application, many more exist. Since then quantum cryptography has been developed that will be secure, even against a quantum computer. Quantum cryptography is already being used by banks and very critical applications, and by the time quantum computers can read secure messages, the Internet may already be using quantum cryptography.