Generally speaking, many English journals require authors recommend two to five reviewers for their own manuscripts. I find several issues exist in the reviewer recommendation.
1. Don't know who to recommend
Some authors, especially those who have never had their papers published in an English journals, feel difficult to recommend reviewers. So they ask the editors to choose reviewers for their manuscripts. Sure, the editors can try to find the suitable reviewers for all manscripts. But sometimes it may prolong the manuscript handling process if editors have to look for reviewers for all manuscripts. So it's better for the authors to recommend several reviewers closely related to the research fields of their papers.
2. Only recommend one's acquaintance as reviewers
In order to have one's manuscript to pass review, many authors only recommend their acquaintance as reviewers. If the reviewer is very familiar to your research and give pertinent comments and suggestions, then it's a good recommendsation. If the reviewer is not quite familiar to your research and give some irrelevant comments and suggestions, it will not helpful to your future research and development. If the reviewer is familiar to your research but still give high comments to your paper even he knows it's not worth it, it is harmful both to the journal and the author. So, I suggest authors recommend reviewers who they are not familiar to and who do research closely related to their research field. Then they can get objective comments and suggestions!
3. Wrongly write the reviewers' emails and other relevant information
Some authors search by Google and casually select the reviewers from the internet. Internet is a good way to obtain the reviewer information. But it needs double checking whether the email address are still effective. One author recommended 3 reviewers for their manuscript, but I found two of them are no longer effective. I have to search the effective email address of the recommended reviewers (judged based on their institution and research fields)on the internet and then have a try. It cost me lots of time for dealing with one manuscript.