Why did Darius I personally review the mercenary troops annually?

This probably increased the responsibility of the military. Darius I personally wanted to see the condition of the troops, since in such a huge power at any moment armed revolts of the conquered peoples could break out.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.