I'd say it depends a lot in your product. If you're a tech company, then perhaps it's best to have it internally. But also, if you're selling a product, then I'd delegate SEO/SEM/Ads to an agency that can handle it better.

When you grow you can have it internally, but maybe at the beginning it's more efficient to have it outsourced.

Another thing you could do is to have the agency train your team in SEO.