In the field of academic publishing, the review process is usually handled by editors. Sometimes, the editor may ask multiple reviewers to review the paper in order to get more comprehensive and objective opinions.

For academic journals such as Optical Express, editors may invite multiple reviewers to review the papers to ensure the quality and academic value of the papers. This will help to evaluate the academic level and importance of the paper and provide valuable suggestions for editors and authors on how to improve the paper.

In the case you mentioned, if a reviewer finishes the review in advance, the editor may think that another reviewer is needed to provide different opinions and opinions. This may be to gain more perspectives and expert opinions to ensure the quality and academic value of the paper.

Please note that this is only a possible situation, and the specific reasons may vary according to the nature of journals, editors and papers. The final decision is usually made by the editor to ensure the quality and reputation of the journal.