What's the point of having hidden input in HTML? What are common uses for this?

I don't see the benefit of having hidden input? If you set the value of the hidden input why not just use that value at the point where you reference this hidden input?

There are reasons for this but I just don't know them.

They're used to pass data that will be needed when the form is submitted. One of the more common cases would be a form allowing users to edit some existing entry. You'll need to know which entry they're editing so that you can update the correct row in the database when they submit the form. The user doesn't need to edit (or even know) the ID of the entry though, so a hidden field works well here.

Other options

URL parameters: This could also be done by building the parameters into the url that the form is being submitted to:

<form action="save.php?entry_id=1234">

but this means you have to handle building the URL properly and escaping the data yourself, and the length of URLs servers will accept is limited so it may not work for longer data. So generally using hidden form fields is the easier way to go.

Session variables: When the edit page loads you'd store the entry ID in a session variable, and then retrieve it on the page that saves the changes. That's a lot easier to mess up though; setting up and maintaining sessions may require adding code in several different places, and then their session could expire in between loading and saving, and you have to make sure it works if they have multiple windows or tabs open, and you have to make sure it doesn't do weird things when they hit back/forward. Because of all these potential pitfalls it isn't a great way to solve this problem--passing the id with the data being submitted is a lot more robust.

Cookies: In many languages/frameworks sessions are tracked using cookies, so they're basically the same solution. The pitfalls are the same as for session variables even when sessions are tracked by other methods though.

It sends additional information that the user doesn't know or isn't interested in (such as a security token) so that if the form is submitted twice, you can compare the tokens and reject/accept that submission.

Not using hidden inputs means the server needs to keep track of these values instead. This requires the server to keep state, which could otherwise be embedded into the request (form) itself. This may make the page RESTless (not RESTful), i.e. break the self-containedness of an HTTP request. If you did keep track of hidden values on the server, you would at least need to embed a unique token into each form to deal with several different unique submissions of the same form. The cleanest way to embed such a token is, drumroll, through a hidden input. :)