Why a tug and barge?

[helen]

I was kind of wondering why goods shipped between the islands use a tug and barge instead of just using a frieghter?

[LikaNui]

I was kind of wondering why goods shipped between the islands use a tug and barge instead of just using a frieghter?
The tug & barge combo uses much less fuel, requires much less maintenance, and also requires far fewer crew, all of which result in a huge saving of $$$.