Fibers

[dsimcha]

I noticed that, in D 2.20, we now have an implementation of fibers for D2.  I
understand the basic differences between a fiber and a thread (preemptive vs.
cooperative multitasking, threads can take advantage of multicore, fibers
can't).  However, from a more practical point of view, I've become curious,
what are fibers actually good for?  What can be done with fibers that either
can't be done at all or can't be done as efficiently/elegantly/safely with
threads?