The Content Decay Detector takes a Google Search Console export and finds three things: striking-distance pages ranking in positions 4 to 15 that are close to breaking into the top spots, pages getting below-expected click-through rate for their position, and high-impression pages generating zero clicks. Each is a different type of content opportunity.
It is a fast way to prioritize which existing pages to update first, rather than only ever writing new content while older pages quietly decay.