From what my college courses espoused, the brain does "defrag" so to speak. No, it doesn't do what Windows does to a real hard drive, but in addition to playing what if, organizing memories is also done during dreams. Remembering the placement of corn and peas on your lunch plate was recall-able before you went to sleep, but the next day, fetching that information would probably be very difficult. A week later, impossible. That's memory prioritizing. It's not know why some snippets get stored permanently but some do. For example, I can remember the telephone exchange when I was just a kid and I rarely used the telephone back then. (It was "Oxford.") Or, for some crazy reason, the square root of 10 to 16 decimal places.