Interesting post & question. I used to work as a freelancer, then worked for various agencies, then started an informal agency, then got most of the way through formally signing the papers to officially start an agency, then hired some agencies, and definitely hired lots of freelancers.
I think the best thing about an agency is that it typically has a locked-in personality and an obvious leverage point. If you can find an agency where you understand both of those pretty well, and you are a match for both of them in any number of ways, you are probably going to be a good customer, partner, owner, or freelancer for that agency.
So it's kind of easy to work with, or around, agencies with that in mind. Agencies also generally try (that is, people working at them naturally try) to avoid making things too serious or personal. I always appreciated that mindset in the way that it avoids some really common business mistakes.
Also I liked that agency employees are generally very aware of their emotions and are able to bring those emotions into their business projects. In larger corporations this can be super risky and is part of the appeal of working for an agency vs. a typical corporation, IMO.