Business and domain skills help in understanding how the system is used by business. Business users have clear understanding of what the correct system behavior is and so the inputs of business users is very important for test oracles. In case of any failure in the software, business users can clearly prioritize the failures, as they know which failure or bug is impacting business the most. So their input is also very important in test designing, risk analysis and defect management.
Good understanding of business processes helps testers to prioritize the features for test execution and also decide priority for the defects. Testers can acquire knowledge about business or domain from having discussion with business users. As business users know which feature are most critical for business to run so they are valuable participants while doing quality risk analysis or defect management. (i.e. they can help in prioritizing the defects)
Business users or domain experts can also help in prioritizing the testing activities, as they are aware of the important features that are most critical for the business. Business or domain experts can also help in providing important test scenarios and test data for critical business scenarios.
As a test manager it is very important that you involve and utilize knowledge of business users as guidance in various testing activities, however you need to be very tactful in utilizing and managing the business resources.
Since business users have other tasks to do, you need to have proper approval for their utilisation in testing activities, otherwise if might become irritating for them if they have other tasks to complete along with supporting test team. You also need to make sure that since business users know about the correct process and software behavior, they do not become bossy and try to dictate testing effort.