Online booking engines work fine for many people, and often offer unbeatable fares. For simple round-trip itineraries it may be faster and easier for you to make your reservations this way rather than through a conventional travel agent.
BUT (yes, you're looking at a big, fat "but") they are just machines, and cannot do the best job for everyone. Yes, a booking engine may be available 24 hours a day, but it will not make tentative bookings, or request seats for lower fares on a waitlist basis, or tell you jokes, or flirt with you. You should call a travel agent if you:
would prefer to trust an agent with years of experience, who knows all the tips and tricks, and who asks all the right questions.
don't have time to enter the same data repeatedly on every different website;
are otherwise frustrated with the online booking tools;
don't trust the internet with your credit card information;
have a complicated itinerary, with multiple destinations, or open-jaws, or stopovers en route;
want a round-the-world ticket;
are totally flexible, are flying halfway round the world, and don't care how you get there, just as long as the price is right;
have different people flying together only part of the way;
have a friend who booked elsewhere and you want to be on the same flights;
are not convinced that the machines have come up with the best option FOR YOU;
have special requests, such as unaccompanied minors, or disabled passengers, or want to take your pet along;
are wondering if you need a visa to visit the country in question;
want additional arrangements, such as rental car or hotel, in conjunction with your flights;
just want to hear the friendly voice of someone eager to please you, and by doing so manages to pay the rent and feed the kids.
A good travel agent earns his or her money by providing the service element that includes the above and more.
By Michael Dickinson JUST Travel - Your English-language travel agency in Germany http://www.justtravel.de
