Yep that is correct. For simple trips you can just use Kayak, but as soon as you start to deal with multi-hop, points purchase and all sort of crazy tricks, you need a service like FlightFox.
Also anything that's not your normal trip. I.e. travelling with pets or surfboards, help using frequent flyer miles, help getting frequent flyer status, or just going to a new country that you've never been before.
"Help me fly with my pet in cabin" would be a great service -- only some airlines do it, and they have weird rules. International even harder due to quarantine rules. (i.e. if I want to go to Hawaii with a cat in cabin, I'd have to charter, since I think the last airline doing cabin pets to Hawaii, Alaska, stopped on that route in 2009).