The AppDynamics REST API to retrieve metrics allows you to get values generated for metrics by specifying the path of the metric and the time frame for the data1. Some of the valid reasons to use this API are12:

To archive 1-minute granularity data. The AppDynamics Controller stores metric data at different levels of granularity depending on the retention period. For example, it stores 1-minute granularity data for 8 days, 10-minute granularity data for 32 days, and 1-hour granularity data for 365 days. If you want to archive the 1-minute granularity data for longer than 8 days, you can use the API to retrieve and store the data in an external database or file system.

To calculate a new metric based on two existing metrics. The AppDynamics Controller provides some built-in metrics such as average response time, calls per minute, errors per minute, etc. However, you may want to calculate a new metric that is not available in the Controller, such as the ratio of errors to calls, or the percentage of slow transactions. You can use the API to retrieve the values of the existing metrics and perform the calculation using your own logic or formula.

To retrieve health rule violations. Health rules are the rules that define the performance and availability thresholds for your application components. When a health rule is violated, the AppDynamics Controller generates an event and optionally triggers a policy action. You can use the API to retrieve the list of health rule violations for a given application, time range, and severity level. This can help you monitor and troubleshoot the health of your application and take corrective actions if needed. References: Retrieve Metric Data, Health Rule API