Papers (if they are KC ones) will give you some reassurance that the breeder takes breeding seriously, in that they want to breed their dog because it is healthy, has a good temperament and is a good example of the breed. If you have papers you can then also check up on the health background of the pup.
If there are no papers there could be lots of reasons, but it may be that they've bred the dog too much, it's originally from a puppy farm, they're cutting costs, haven't got health tests done, etc..
The papers give no assurance that all these things will be fine, but usually show some responsibility from the breeder.