In S/4 HANA, to simplify the business process, every vendor and customer is defined as a business partner.
Business Partner eases the master data maintenance and helps the data to be harmonized.
Business Partner reduces the redundancy of the data based on different roles a business partner can carry out.
Business Partner (BP) helps the business folks to consolidate the earning and payable together so that they can get a single report of earnings and spending based on a single entity.