I can speak to the Treasury Board standard on security screening, which outlines when an employee may need to be suspended. Speaking generally, it's when they would represent a potential security risk to the organization—to people, information or assets—or to the conduct of the investigation.
Again, speaking generally, if someone is under investigation in the workplace and still has access to witnesses, they may try to influence them. That would be a concern, so they may be removed from the workplace to protect the integrity of the investigation.