I've heard some people wheel them round in prams etc which is more than a little odd. I don't collect anything though so I obviously don't "get" it. I'm sure most are bought for the same reason people buy china dolls or steiff bears. No harm in it.
Someone mentioned dolls with heartbeats. Some reborners put small heartbeat boxes which they put in the dolls nappy, and also small hand warmer type bags which you can reuse to make the baby warm (I don't know why you'd bother with that to be honest)
You can buy newborn type reborn doll with hand moulded umbilical stumps! (again, not sure why this would appeal)
There are lots of individual touches the dollmakers use: I used to put "chicken fillet" squidgy bra doodahs in the tummy and the bottom to make mine extra cuddly.