If a new functional mutation occurs in an individual, and is subsequently propagated from generation to generation, then a particular ancestral haplotype of neighbouring polymorphisms will be associated with that new variant. The haplotype will become shorter with time, due to recombination, and unclear, due to mutation.
This is helpful, in that one does not have to genotype at the mutation itself to infer whether or not they carry a particular variant.
This is unhelpful, in that polymorphisms may be risk factors for a disease without being causative.