Pretty much every journal requires authors to suggest potential reviewers at the time of manuscript submission. Some require four. Others requires as many as six, which can be a right royal pain in the butt. But it's required. Supposedly to help the overworked editorial types of the journal find the right reviewers.
But guess what? I try hard not to use them.
I'm on the editorial board of a middling journal and am often looking for reviewers. But I try not to use the suggested ones. Why? Well, all too often authors suggest people they shouldn't. Like co-authors from previous papers. Collaborators. And the like. I find I spend just as much time trying to figure out whether a suggested reviewer is a kosher suggestion as I would just identifying new ones of my own choosing.
Okay, so I suspect the above is a rarity. And it's certainly fairly easy to confirm the correct email address information for a suggested reviewer. But still...
Finding reviewers isn't easy. By that I mean people actually willing to review a manuscript. So sometimes I do have to resort to using a suggested reviewer or two. But I sure as hell make sure they're relevant suggestions.
The upshot is, if you have me as the handling editor you may be better off putting down people you don't want to review your manuscript as suggested reviewers.*
* This is an attempt at humor.