2 review periods for the same timeframe

Summary

Is there any benefit to having the a review period linked to each performance document?

Content

We currently have 2 performance documents. Each one has it's own goal plan and review period. As we don't use actual goals in the goal plan, I am questioning why it was set up this way. Everyone keeps saying it has always been this way. 

Can anyone help me explain the benefits of having this set up where each one has it's specific eligibility.

Review Period A , Goal Plan A Eligibility, Performance Document A Eligibility

Review Period B , Goal Plan B Eligibility, Performance Document B Eligibility