There is also timeliness, as often freelancers tend not to respect deadlines as they should. Price is also a factor, as an agency may be able to provide volume discounts that are not available when negotiating with individual freelancers. Also, some editors don't like to deal with fielding calls from numerous freelancers, and prefer to deal with an agent. Editors like variety and the freedom to be able to use different writers who are experts in various fields. An agent works as a mediator between a writer and editor.
Writers tend to take a rejection more personally from an editor.
A good agent moves on to the next editor and refrains from passing negative remarks back to the writer. To an agent, it's simply a reflection of an editor's personal style and taste. There are publications that choose to ignore copyright laws. Do not send original negatives. This enables writers to protect their own material.