The matrix organisation is used mostly by mapping matrix manager in the relationship. You can map the position in the matrix relation ship. This is mainly used in PMS , where an employee is evaluated by his/her manager and then by Matrix manager or can be other way round also. This helps employee to get the perspective of both the manager.
Similarly you can use this relationship in many workflows where in matrix manager need to know or approved the actions related to employee , may it be related to onboarding , cross boarding or offboarding and even in talent management decision.
Hope this helps you , in case you need more information , will be able to help you on that