With modular PSUs all (or nearly all) connection cables are removable and only need to be attached when there is something that requires them.
For example, with a Hard Drive and a DVD-RW, you might only need one lead, however, if you were to add a second DVD-RW and HDD you would just attach another cable to the PSU (usualy by means of a push-in connector or a screw-on type).
This PSU design obviously could lead to decreased clutter inside of your PC which could in turn increase airflow to all of the components potentially increasing lifespan and at least cooling during that lifespan, the removal of spare dangling leads also makes the inside of your case look nicer, especially if it's windowed
