It is much easier and less time demanding than having to configure all the parking positions before you start the flight.
It's clearly much easier to configure an airport ONCE in your life (considering an airport customization will even survive a reinstall of both GSX and the sim), than having to revoke the parking system *each* time you use it.
And, maybe you are not aware that, every modification you do to any parameter in the parking customization page, can be applied to a single parking, but also to many parking at once, or even the whole airport at once, depending what you have selected on the left side tree structure.
So, for example, removing *all* marshallers from an entire terminal (because you know there are 3rd party docking systems there), just require clicking the terminal, disable the Parking system and click Apply, which is just 3 mouse clicks, only the first time you use an airport, instead of having to repeat the same procedure each time you use GSX there.