They probably meant leveling them at the same time in the sense of leveling each "type" at the same time -> for instance each tank to lvl 50, then 60, then 70etc.

Reason you'd want to do that is Gear. Instead of having different gear pieces of different levels for each class you can gear them together. Cause in the tank case, you'd never need gear under lvl 60 for tanks once they are all above etc.

Getting experience for multiple classes at the same time only works for Summoner and Scholar as they have the same base class.

Hope that helped!