Kapacitor is a Continuous Query engine designed for use with time-series databases, allowing developers to run long-running queries in real-time without impacting system performance. The tool was developed to address the limitations of traditional query engines and provides an efficient way to manage continuous data processing tasks. To determine which tool best serves a given use case, developers should consider questions such as whether they need real-time data processing capabilities, how complex their queries are, and what level of performance is required. By understanding these factors, developers can choose between Kapacitor and other tools, like InfluxDB, to meet their specific needs.