Why is <form> tag useful when we can directly use <input> tag?

Answer 1

Form groups all the inputs and submit them to server.

Answer 2

It's standards pedantics.
The statement is legitimate as far as the standard goes: in HTML 4.01, the definition for <form> specifies that it may only contain block elements or <script>. As far as what every browser in the world allows, it's fine.
I can only guess that they consider <form> to not be a layout tag at all, and they want all inline elements to be contained inside a block element.
Yes, you're supposed to place a <div>, <table>, <p>, or some other block presentational element inside the <form>.

https://stackoverflow.com/questions/5439078/why-is-an-input-tag-not-allowed-directly-within-a-form-tag

https://www.quora.com/In-coding-why-do-we-need-to-use-the-form-tag-to-put-an-input-in-HTML