Why journals ask for potential reviewers' details during submission of a manuscript?
The editor, who selects the reviewers, may not be familiar with the particular sub-area, when a journal is not hyper-narrowly focused. In that case, providing a list of potential reviewers directs the editor's attention to possible scholars in the area. Once your attention has been drawn to an individual, you can spend some time discovering whether they would actually be good in that area.