It's effectively a form of what we might now regard as a statute of limitations. With the Norman Conquest in 1066, there was a huge upheaval, and obviously lots of formerly important people lost land, rights and property. During the initial period after the conquest, the law was basically a combination of the King's word and what you could get away with. Later, when concepts like property law and courts independent of the King's whim at the time became a thing, they didn't want to have to deal with all these ancient grievances, so set a date, the start of the reign of Richard I, and deemed anything that happened before that to be "time immemorial" and therefore not subject to the legal system. If you can prove you owned something or had a legal right on that date, anything that happened before would not count. The date was set something like 80 years prior, so things in the relatively recent past would still be included, but things like "your great grandfather stole this field from my great grandfather" would not.