When to book tickets from travel sites rather than airline sites?
When booking plane tickets, I generally buy directly from the airline's website (as it is often the lowest or near-lowest price for the destinations I travel between). However, I know there are many other travel sites like Expedia, Travelocity, or Kayak.
When does it make sense to buy from one of those sites instead of direct, and what are the pros and cons of tickets purchased through them?
For domestic flights in the US, the cheapest price is almost always on the airline's website. Additionally, airlines sometimes include other benefits like additional frequent flyer miles, reduced baggage fees etc. for booking directly through their site.
Online travel agents such as Expedia, Travelocity, Orbitz etc. can sometimes find flight combinations that you cannot book directly through a single airline. This can be useful when there is no single airline (or airline alliance) that serves your route. This is not common for domestic trips, but can happen sometimes for international flights between more minor destinations.
There is also the convenience of using a single site with all your information already stored instead of having to navigate a different a airline site every time you make a reservation. This can be especially useful for foreign airline websites which are sometimes more difficult to use and can be troublesome with foreign credit cards.
Finally, if you book through an online travel site, they become your "agent" and can take care of things such as cancellations, schedule changes etc. You might find it more convenient to speak to an English speaking customer service agent on a 1-800 number than calling a foreign airline which may not have a toll-free number or even a number at all in the USA.