Better understanding and visibility of the entire business process
Better error and exception handling
Increases the effectiveness of your business
Allows you to improve your business processes
For more information, Refer, follwoing guide, secion- Benefits of business process management.
7-1_Getting_Started_with_Business_Process_Management.pdf
IMHO, the benefit that BPM brings over a direct service invoke is clarity to the business, which is largely and often overlooked. BPM allows the business and IT to communicate in a similar language, providing higher signal fidelity (reduced loss of message).
More over, the Software AG webMethods Toolkit, especially the newest release, provides a more complete environment, keeping more of the assets within a single environment.