DNA from one individual is not used to determine consensus genomes anymore, if ever. Many individuals genomes are sequenced. Go to UCSC genome browser and view the most recent human genome build (hg38). Look at your favorite gene, view SNPs. In any single gene, a few single base pairs will vary. These are mapped to the reference genome. It is also very important to note that if you are a physician for example, and you have your patients genome sequenced by the hospital genomics core, the core needs a reference genome to map the patients sequences to so we can easily see where the reads fall in the complete human genome and if there are any significant mutations (done often for cancer patients). Having even a single human genome, fully annotated, to serve as a reference for mapping new sequences to, is ridiculously useful, because humans aren’t that different.