I think one of the advantages an agency/freelancer can bring is perspective and experience that comes from running multiple accounts. You can’t get this when you’re flying solo.
I’m a Google Ads guy so I can’t comment on FB specific stuff but I know that in Google Ads there are patterns/tactics that work well across most accounts. I can apply these while setting up a new account and often get leads for my client within a few hours of enabling the campaign. Starting from scratch, on your own means you have to learn this yourself. It can take a lot longer and cost much more – if you ever do.