One of the important data in power monitoring and saving is to measure current flow. Knowing your application current usage allows you to provide data such as power measuresment. And with current measurement you can implement current protection to your circuit. Not to forget many other features that you can implement with current measurement including: battery chargers, switching mode power supplies, digital watt meters, programmable current sources, etc.
This ACS721 current module is based on ACS712 IC, 5A version, which can accurately detect AC or DC current. The maximum AC or DC that can be detected is 5A in both direction (for AC), and the present current signal can be read via analog IO port of Arduino. 5A is good enough for current measurement in many applications, example LED (~20mA), medium brush motor like SPG30 or SPG50, and even solar panel.