Life as a Web developer can be hard when things start going wrong. The problem could be in any number of places. Is there a problem with the request you’re sending, is the problem with the response, is there a problem with a request in a third party library you’re using, is an external API failing?
Good tools are invaluable in figuring out where problems lie, and can also help to prevent problems from occurring in the first place, or just help you to be more efficient in general. Command line tools are particularly useful because they lend themselves well to automation and scripting, where they can be combined and reused in all sorts of different ways.