How to build a REAMP box and WHY you need one

What is a reamp box, why would you use one and how to build it for about 20$. Schematics: Parts list: Box: Hammond 1550A or any metal box bigger than 89x35x30mm XLR Connector: Neutrik NC3FAV1 (any XLR connector will work) Jack Connector: Any two contact mono jack in plastic housing (like Neutrik NMJ4HF-S) Transformer: VTX-101-1604 (or any 600:600ohm line level audio transformer) Resistor: 10kOhm (250mW power rating is enough) Potentiometer: 100kOhm Logarithmic