With the development of the internet and self booking tools, travel agents add little value to simple "there and back" bookings. As an agent I would be the first to admit that.
Indeed, for a travel agent, this sort of booking is the equivalent of asking a nuclear physicist to change the battery in the canteen clock. If you know what you want on this sort of trip, it is much more cost effective to simply go on line and book it - or use the self booking tool.
As long, that is, as you have the time and facilities to check (on a trip, say, to Paris) about three website and a self booking tool and having checked them, hope that the first set of flights you saw are still there when you go back to book them. As long, that is, that you then check the fare rules to make sure that the fare is sensible and then check out to see if it is better to book a return or two one ways.
As long, that is, you have the time to do the above, in between meetings, from the back of a taxi, running to catch a train. And, of course, you have a good internet connection that doesn't fade, or stop working as the train has just gone into a tunnel or your ride has entered one of those "dead areas" - and you have time to start over.
We agents still have a role. We know you. True, it may take a short while to understand your likes and dislikes but once that is sorted and understood, we can make your arrangements all from a telephone call (or two).
Some airlines, on return tickets, do not allow any changes at all. Book two one ways, and this restriction is lifted - and it may even be cheaper. There are also some very unusual fares that, if booking on a cost factor alone, means that you can waste a lot of money. Lufthansa offer two types of Y (looks like full economy) based fares to Frankfurt. Both these fares have a cost situated in the upper £400's. The cheaper (and only by about £6) allows no refunds, the other does. Get it wrong and saving £6.00 just cost you £400. So, despite looking simple and easy to book on the web, there are still many traps set to catch the unwary.