"custom alerts" in Google Analytics let you watch out for specific things that you know you're interested in. "Custom alerts can be used in many ways that it's sometimes hard to know where and when to incorporate them into your workflow. This video shows how to use custom alerts as a campaign management tool. The idea is that, when you set up a new campaign, you set up alerts that help you manage the campaign -- for e.g., alerts that trigger when revenue from the campaign increases or decreases," stated Google.
Campaign management is just one of the ways you can use custom alerts.