Why do people use travel agencies? For me it would just be another layer adding expense, mistakes and complexities. I'm guessing people use them thinking they will reduce those things.
It makes life simpler in cases when flights or hotel gets cancelled and agency is responsible for handling things.
However, even though I almost always fly with just a small bag, I still might hesitate taking the 3rd party service because they insist on doing stuff like checking in "for me", which I don't need (since that is a trivial process), and I have experienced them not sending the boarding pass in the time I arrived at the airport.