It combines both DEP and VPP in one place. It supposed to make it easier to manage the deployment without jumping from the different apple programs. It also makes it easier to create Apple IDs for employees. Think Apple School Manager for business.
Apple Business Manager lets organizations buy content and configure automatic device enrollment in an MDM solution. Apple Business Manager is accessible on the web, and is designed for technology managers and IT administrators. New enhancements from DEP to Apple Business Manager include:
• Support for Administrator account and multiple Manager accounts
• Any Administrator/Manager account may now accept new Apple terms & conditions
• Create and administer Managed Apple IDs for end users
• Assign Roles to end users
• Manage locations
• Purchase/Manage apps and books without separately managing VPP for Apps system
• Transfer licenses for apps and books between locations • Access activity log files
https://help.apple.com/businessmanager/#/tes40577306d