Adding a monetary field to a Model Odoo has special support for monetary values related to a currency.

it's automatically mange the currency position(after/before ) and currency symbol  

usually while creating a fields that unit of measure  have linked with  currency ( Like amount_total,price_subtotal,payment_difference.)  we user Monetary for that field.

The widget='monetary'  us used with such type of field which automatically  shift the currency  position(after/before)