for what the name attribute is used for?(inside input tag)

Answer 1

For proccess input value.
In server side for access to value with name

Answer 2

name for called your PHP file

Answer 3

ex: <input type="text" name="user" placeholder="Enter username" />

when u will try to retrieve user input data you must need to use like this... to sanitize or work with them

$_POST["user"] / $_GET["user"]

that's the use of name attribute .