Most agile teams do sprint retrospectives at least once a month, to iterate and improve on their software development process and workflow. However, a lot of those same teams rely only on their feelings to “know” if they have actually improved. But you need an unbiased reference system if you want to compare how two sprints went.
Depending on what you’re focusing on, there are metrics that you might be interested in. For instance, if your team uses estimation, tracking how those estimates pan out could be worthy indeed, and comparing the variance across sprints could provide such a metric.