For Facebook specifically, is very true is becoming a lot easier but would say agencies have the following benefits that you would struggle to get without them:
* Support from FB. How many people makes posts on this sub saying they can’t get access to FB Support? For agencies its a given and especially the FMPA ones, you get access to much wider parts than just account reps (marketing science / solution engineering etc)
* Ad approvals. This is very real especially right now in peak season. You can basically get your ad accounts approved in “flash sale” mode, so all ads get passed audit no matter what. Again a perk of an agency.
* Alphas / Betas. Agencies will generally speaking get access to the more newer elements of Facebook first unless you are a very large brand running in-house. Things beyond Ads Manager like CAPI & FB Attribution also are important to know how to use.
* Technology. The smarter agencies that aren’t just full of account managers actually build on top of Facebook via the Marketing API, which is where you can do very interesting stuff that can drive incremental value on FB.
If you are measuring based on Ads Manager numbers and you go broad / Power of 5 / whatever FB are saying to do, is true it will probably perform. But if you take a step back and look at the bigger picture, then this is where it gets complicated which is where an agency can help.