I believe the major benefit is that these article will receive elevated attention from a very specific target audience who are interested in the featured topic/focus area of the special edition.
The converse is the drawback, in that general readers of the journal may overlook special editions when these do not appeal to their interest areas.
Personally, special issues benefit authors who have articles on the specific issue. However, giving special attention may distort the existing problem from the surrounding. It does not give the authors to chose the issue by themselves, rather they may be inclined to that special issue.