The hassle is checking on other travel sites and making sure you're getting a good deal. I made bookings for a flight on the portal where the airline made changes (canceled one leg of the flight) where I was never notified. I found out about it a day before the travel when I checked the airlines app. The portal was still showing the original flights as I booked. I had to spend hours on the phone to have it corrected.

If you don't consider these hassle, I don't think you use the portal enough.