You would take off the seals and add fresh grease or something like Phil's Tenacious Oil in the bearings of the freewheel. If you are running a belt you don't need to lube it though you can use silicon spray however you would lube a chain. If you are running fixed you would just need hub maintenance or replacement of bearings as needed and same stuff with chain or belt.

Basically anytime you have metal on metal contact you want something keeping it lubricated or protected in some way from rust and corrosion and to keep everything moving properly if needed (as in the case of your hub). It is always better to err on the side of a little extra maintenance then none at all. Alfine hubs are not the most expensive thing in the world but I wouldn't want to have to replace it because I didn't change oil once and while. Just like you need food to survive a thing with moving parts needs lubrication.