How to use "what's the use of" and "it's no use"?

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What's the use of (doing something)?  

It's no use (doing something).

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These expressions are ...

"... used for saying that something is not likely to have a successful result, and so there is no point in doing it". 

"...used for saying that there is no point in doing something because it will not be successful or have a good result." 

What's the use of worrying about it?

So what's the use of trying to prove my innocence?

It's no use asking me. 

It's no use - I can't persuade her.