This can be extremely useful in some cases.
As an example, I filled my inventory with 28 Porter IV (1 worn, 27 in inv) and went granite mining. This means I banked 28*20=560 granite without having to actually go to the bank at all. This also works in combat, where some (but not all) drops will be banked. Right clicking a Porter will have an option "Check items", that lists everything it will auto-bank for you.
There are six major fields of items it will bank, from the skills: Farming, Fishing, Hunter, Mining, Woodcutting and "Other". The last category holds an assortment of different items.
The Porter, when worn, will occupy the Pocket slot.
The Porter is not tradeable.
This table shows the Porters available in game: