Employers often require it.  Some advantages for the traveller:

* If something goes wrong, it is up for the travel agent to solve.
* Travel agent will bill employer directly, so that employee does not need to claim potentially large out-of-pocket expenses back later.

See also on The Workplace: [*What reason does an employer have to mandate all travel booking be via the same travel agency?*](https://workplace.stackexchange.com/q/6846/1898)