Why do we need ajax/jquery calling for colntroller method when we already have default functionality given by MVC?

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C#

Ajax

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asynchronous

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Suppose in a sample application there is a SUBMIT button, and in FORM tag I've alreasdy set POST method. So when I click on submit button it'll automatically execute respective controller's action. This id already there in MVC. Then why do we need AJAX or jQuery for the same purpose, i.e. calling respective controller's action?

I know Ajax call would be asynchrounous, but other than that any other advantage?

What I have tried:

I know about AJAX asynchronous behaviour but we don't want it always, in that case what should we do?