Web challenges in CTF competitions usually involve the use of HTTP (or similar protocols) and technologies involved in information transfer and display over the internet like PHP, CMS's (e.g. Django), SQL, Javascript, and more. There are many tools used to access and interact with the web tasks, and choosing the right one is a major facet of the challenges. Although web browsers are the most common and well known way of interacting with the internet, tools like curl and nc allow for extra options and parameters to be passed and utilized.
To-Do (need a website/server)
If you are running linux and want extended functionality (like passing custom headers) in web challenges, bash (terminal) commands are your best bet. cURL is a simple but extensible command-line tool for transferring data using various protocols, and allows users to use HTTP to interact with servers, including POST and GET methods.
To see curl at work, you can simply run curl 8.8.8.8 (Google), and the html of Google's home page should appear.
There are many other options and flags that can be passed to curl, making it an extremely useful tool in CTFs