I just don't understand why it is set up this way. What is the purpose ? I have read everywhere and tried to understand that but could not find the answer 

If i use quickbooks just as a bookkeeping tool for records. Not to process payments. 

1-Why do i have to issue a credit memo ?? What is the purpose of that ?

2- if i made a sale of 100 dollars then refunded the 100 dollars, why does it show -100 dollars at the bottom ?

3- and why issuing a refund by going to + new ..refund receipt.. it does not reduce the balance of the invoice ?