Someone asked a similar question on LinkedIn “Why pay an accountant to check your maths?”.
You pay an accountant for the things that you do not know.
You pay a DPO for the things that you do not know.
However, if your DPO is just into the theory they are not going to be of much use in the long run – they need to be able to demonstrate that they understand your business and the issues you face 0 not just data protection legislation.