How do you build applications to be scalable and have high availability? Along with developing the applications, you must also have an infrastructure that can support them. You may need to add servers or increase the capacities of existing ones, have redundant hardware, add logic to the application to handle distributed computing, and add logic for failovers. You have to do this even if an application is in high demand for only short periods of time.
The cloud offers a solution to this dilemma. The cloud is made up of interconnected servers located in various data centers. However, you see what appears to be a centralized location that someone else hosts and manages. By shifting the responsibility of maintaining an infrastructure to someone else, you're free to concentrate on what matters most: the application.