To prove they didn't discriminate against you in the hiring process, and to use as a basis for comparison to see if your story changes when/if you reapply later.
Otherwise, assume they will not dig it out when an opening appears. Looking them back up if any time has elapsed is a waste to the employer because people get job, people move, etc.
Keep your job history etc on a three by five card and use it each time you apply for a job so if you reapply it is consistent.