Events can be triggered by a number of different user interactions. Sometimes Google analytics is unable to track goals using the other goal types. When websites contain I frames, certain clicks or actions are very valuable but can be difficult to track moving across domains.
Auto Advertising is a great example where we see this very frequently. For typical automotive websites the various tools that they use are I framed into the website so that the website manufacturer can churn out more websites at a lower cost. This makes it very difficult for the dealership to see which traffic sources are actually delivering the best traffic and leads for the dealership. Utilizing Google Tag Manager, a website can also track the mobile clicks to call from users that visit the website through the event goal type.