This feature solves a common problem for developers and system administrators. Over half of all salesforce.com transactions are API based. At a billion transactions per day, that's a lot of API calls. As with most things that voluminous, there are limits to help protect the service for everyone. If you run into one of these limits, you may be prevented from continuing, which can be a problem if you're in the middle of a nightly data load or are using a mission critical application that relies on the API.
API usage notifications sends an email when you have hit a configurable threshold. For instance, you might want to know when you are 50% of your API limit threshold and again when you are 80% of your threshold. This information can help you prevent a work stoppage, develop some simple forecast capacity planning, and react if you're about to hit a limit.
Monitoring usage is critical to the health of your business applications. This is just another way you can get what you need to keep your mission critical apps running.