Basically, if you are trying to do Flame Leviathan 4 towers the proper way for the first time, you will find that you wipe alot in a short space of time, taking heavy gear damage quickly. Your vehicles are affected by the item level of your gear, but if your gear is broken it will not affect how much you hit for. Therefore if you let your gear durability drop to 0%, and continue your progression on the boss, it will make no difference to your damage output or vehicle Hitpoints, but will prevent you from taking any further durability loss therefore saving you money.
I know there is a brilliant guide for doing Flame Lev four towers easily, but there is a chance you can get banned for it, and there is not much sense of self achievement in doing it this way in my opinion.