What is purpose of assign value in the function?

Question

I'm a BEGINNER so try please explain with simple words.

The source of my code

What is the purpose of line 3 here:

function sumAndMultiply(a,b,c) { 

        var sumAndMultiplyArray = []
    
        aPlusB = sum(a, b)[0];   //line3
    
        var finalSum = sum(aPlusB, c)[0]; 
    
        sumAndMultiplyArray[0] = finalSum;

        return sumAndMultiplyArray;
    
        }
    sumAndMultiply(4,7,5);

Is it is wrong to make it like that:

function sumAndMultiply(a,b,c) { 

        var sumAndMultiplyArray = []
        
        var finalSum = sum(a, b, c)[0]; 
    
        sumAndMultiplyArray[0] = finalSum;

        return sumAndMultiplyArray;
    
        }
    sumAndMultiply(4,7,5);

Answer 1

The sum function just receive two parameters, the third parameter c will be ignored and won't be summed up.

EDIT:

The purpose of line three is to tell the computer to save the resulting value of calling function sum with params a, b into a new variable called finalSum. This can be achieve in one line, but you should know that ease of read is way more important than compact code.