In what scenario would you use a custom metric?

Custom metrics are used to track quantitative data that Google Analytics doesn't automatically collect. Tracking the number of minutes played for each video is a perfect use case for a custom metric because it's a specific numerical value tied to an event (video play) that isn't a standard GA metric like pageviews or sessions. "To find out the average number of pageviews per user" is a standard calculation available in Google Analytics reports (e.g., in Audience reports, or by creating a custom report with these metrics). "To see how often a page was the landing page for sessions including that page" is also a standard dimension/metric combination available in the Landing Pages report. "To record the title of a video watched on your site" would use a custom dimension, as a video title is descriptive text, not a quantifiable metric.