It's pretty straightforward and simple: A travel pack cube is a a small, water-resistant, light-weight pouch that can help you organize and segregate items INSIDE your travel bag.
It's a non-intrusive cube that saves you even more space than the classic organizers for your luggage and bags that tend to take up more space than your actual stuff.
Imagine going camping or travelling to an outdoor site: Mobility is key for the activities you will be experiencing. You can't be hassled by the bulkiness of your organizers and most importantly, you need to travel light. Only the essential clothes and gear are the things you need. You're not going on a fashion show and ramp all the way in the woods now, would you?
A packing cube in your bag even in an urban or city setting is also handy. You never know when a sudden drizzle or rainfall would come, and with the pack cube you're at peace knowing it's safe from the elements. It's water-resistant, but not waterproof so please don't try dipping in a pool with your gadgets.
In summary, a travel pack cube enhances your travel bag, supplementing it by maximizing the space of what goes in and out of the luggage.