The human resource staff member will be knowledgeable about HR policies and procedures.
If the incident involves an employee, the HR person is the best resource to advise on policies that were violated or that have to be adhered to, before taking any action on the employee.
For instance, if an employee has been found guilty of theft of confidential data, the HR staff member will advise on the next steps that can be taken keeping in mind HR policy compliance.
While the HR role is primarily after the incident investigation, and mostly only when an employee is involved, it is important to have them on the incident response management team.