Waypoints
I used to play video games a lot as a kid.
My favourites were (relatively) open world games like Final Fantasy and Zelda. These often involve crossing vast landscapes to complete missions, battle monsters or collect powerful weapons. While traversing the various terrains you might discover a chest but not yet have the key, or encounter a monster too powerful to defeat at that stage of the game. This could happen multiple times in a single journey across these worlds.
Some of the games allowed you to set waypoints on the map - a marker that would make it easy to find your way back to what you had discovered earlier in the game. Rather than trying to hold all of this information in your head until after you have that key or have levelled up, a waypoint provides a direct path back to something discovered in the past.
Writing, and building a Personal Index for me has like setting my own little waypoints. I'm setting little markers around an idea or concept which I can return to when I have more information, or when I want to use it. That might be years from now, when my experiences and knowledge have grown and I can add further insights, or when my ability and need to use that information comes around.
These waypoints are like little investments for my future self in the game of life.