What if a person doesn't remember to turn off their personal results when doing a search? Then the results will be influenced by sites they have visited. In that way, the results are tailored to each person's own needs, rather than the more unbiased and impersonal search results you get when you do a search on Google search without personal results enabled. So all that time spent by bloggers/webmasters doing SEO is for not as relevant considering sites that don't even deserve to rank so highly are included in top results, just because that ONE person visited similar/same sites before. So in that way, the individual is creating their own search results based on their own needs.