Finance invoice Accounting - G/L and reconcillation accounts

Hi Experts,

I have a functional question related to the determination of accounting of FI & MM Invoices. I have reading the forum but I still have a conceptual understanding problem.

On FI side , When you entered an invoice i.e FB60 you should enter the GL account, additionally through the customer master data SAP obtains the reconciliation account. In this scenario you are recording the expense in the reconciliation account and in the specific GL account. why this happened twice? is not enough with the reconciliation account? or the GL account is for Cost control?

on the MM side, is the GL account obtained from the PO or can be changed when the invoice is entered through MIGO? here again if the GL is needed is for cost control purposes?

Regards,

CC