Bear in mind OTAs may not have inventory at all. They often just offer a discount on what they're paying - it's quite instructive to buy an Opodo prime ticket deeply discounted on say EasyJet or Ryanair and look at the receipt details which give the actual purchase price. It'll be on a random credit card and be the actual selling price of the ticket from the airline.

What they do to generate profit is to route you onto their site for bags and seats and various other ancillaries including insurance and special customer care options, which they then sell at a stonking margin. I suspect insurance and customer care are the moneyspinners, people are often nervous and will sign up to this stuff.

Ryanair in particular really hate this, and they'll often make you do a validation process to be able to manage the booking yourself. Because they want the ancillary revenue and resent someone else getting in the way of the direct relationship.

I guess this is effective because these companies exist and seem to make money. It's very different from a full service travel agent which would be expected to add some value in the interface between customer and airline. Airlines mostly seem to want to eradicate the intermediary now though,

The trick is to take the loss leader fare and nothing else. That's where discounting OTAs are well worth considering as part of a low cost travel strategy.