I agree that it's not a necessary thing, but some kids are born with a strong sucking urge and do not care to comfort nurse. This applies to my DD. For the first five weeks, she'd nurse and nurse and nurse from around 7PM to 2AM. After about an hour or two, she'd latch on, suck for a few moments, then arch her back, pop off, and scream. Frantically search for the nipple again, latch, suck, pop off, scream, repeat. She didn't like fingers (ours or hers) to suck on. Eventually, we got her a pacifier, and it helped things out a lot. Once she'd finished nursing (repeat the latch, suck, pop off, scream routine a few times), I offered her the pacifier. I got a weird look, then she sucked a few times and went off to sleep. I was amazed.
So no, pacifiers are not necessary, and probably should be avoided if possible. But in cases like mine, they can be useful.