IaaS is how you can complement or replace your on-premises hardware with virtualised systems in the cloud.
What's wrong with using your own infrastructure? Consider how much time it takes to specify, order, take delivery of, install and configure a physical server. A few weeks? Then think of the effort to maintain it, and securely dispose of it at the end of its life.
How long would a similarly specified Virtual Machine (VM) take to install? A few minutes. When you're finished with it, you simply delete it.