And the other thing to consider is that 3rd parties who sell airline tickets cheaply do so by cutting their margins to the bone by providing the minimum of after sales service for free. Often this will mean they add their own admin fees attached for any after sales service which can typically be executed poorly by badly trained staff who are typically at the pointy end of passenger frustration when they discover that the after sales service is shockingly poor and comes with additional fees.

If it's a slam dunk that you will not mess up your booking online with silly errors and you will definitely fly what you book with no need for any changes and you can extract a sensible discount by booking through a 3rd party then proceed, albeit with caution having weighted up the risk/reward of that choice. But most seasoned travellers will be better off booking direct with the airline so you can get some semblance of after sales service.

You can only expect to deal with the airline direct if there is IRROPS on the day.

The main exception to this will be business people who are forced by policy to book via a corporate travel agent, but their company are paying for an after sales service.