Question: What is the difference caused by using “nope” instead of ”no”?

Is it used because “nope” sounds better and not straight like “no”? In some situations, it feels like *nope* is better to use than *no* even though it adds 2 more characters. What is the reason why many use it?

Answer: 
"Nope" is informal. You wouldn't usually want to use it, for example, answering questions for a job interview, but it would be an appropriate answer to a friend's question "Have you seen that movie yet?"
*Nope* is an informal variant of one of the meanings *No* which is:

> 
> 3 : not so —used to express negation, dissent, denial, or refusal (**e.g.** no, I'm not going)
>    
> [Webster-Merriam](http://www.merriam-webster.com/dictionary/no)
> 
> 
> 

That is why, we can only use it for *Yes/No questions*.