Feedback is a key part of authentic cranked-amp tone, and this technology enables adding feedback at private playing levels.
Sustainers are also highly controllable.
The headstock model is bulkier but enables using the neck pickup with the sustainer circuit engaged. Now both electromagnetic and acoustic circuits are available, as add-ons for your existing guitars. It makes a great sound source for loops. What I remember about the old sustainiac is that it required external hardware and some large device connected to the headstock of the guitar. Here is a site that has some samples of sustainer technology.