It's a combination of things.
Digging in the scraps to be composted is work, Trying to avoid that as much as possible.
When the ground is frozen... It's normally not a foot or so down.
The Jugs hold more scraps than I'd be able to
compost by digging in bed, especially while I have plants growing.
The worms distribute the nutrients to where the roots are accessing them.
It's a quick and easy way to vermicompost. Certainly not sayin' it's the best.
-Ryan