To answer the "how," the larger part goes inside the vagina to provide internal stimulation, usually either by vibrating or rotating, while the smaller piece vibrates against the clitoris (men have also used them to stimulate the prostate and testicles and/or perineum at the same time, but I don't know how common the practice is).
As to whether or not they're better than other types of toys, that really depends on the user. A lot of folks find them to be very stimulating, and having "two toys in one" can be considered convenient. For others, they just don't really do much, or have trouble finding one that works for them (finding a rabbit-style toy that fits your anatomy can be tricky).
I myself currently own four: one that's just "meh," one that's pretty good, one that I haven't used much yet, and one that's my second-favorite vibrator. I'm all over the map.