I mean its just a practicality.

Lots of governments just end up with career lawyers and politicians.

They can choose their advisors and their advice.

The more varied experts you can get into parliament, the more likely you have an on the record disagreement from an expert preventing a politician from choosing the incorrect advice that suits them better.

Thats not to say they are going to be any better for corruption, but in todays world there are so many deep silos that governments just completely step over and disregard.