I think the point is that there is no reason for a getFirst(iterable) in that this could be done with iterable.iterator().next(). Guava makes an excellent attempt to keep the API small and so does not add things that could / should be done easily another way.
On the other hand, there is not already a mechanism to test if an iterable is empty and if so return a default value instead of the first value. Hence, getFirst(iterable, default).
Also, there is not a simple way to get the last element, hence getLast(iterable) and getLast(iterable, default)