In games that require a great deal of investigation, it can often help to keep detailed lists of clues, unexplored leads, and witness comments. Otherwise, you might not pick up on the contradictions between two witness statements or you might forget that red hot lead about the diner, and that can quickly and steadily grind the game to a halt and leave you scratching your head and feeling frustrated. If you're playing a political game, it also helps to jot down the relationships, likes and dislikes discovered over the course of the gameplay so you know who to target and how to target them.
The amount of notes you take, of course, should depend on the game itself and your own play style. No point burning yourself out taking lots of notes if you hate taking notes when you can just jot down the most important, and forgettable, details.
So let's assume that you've dutifully kept good notes from session to session (you may be able to wrangle extra experience points out of your Storyteller if you offer to do this). What do you do it with it now? Well, often times it's enough to simply sit back and re-read over your old notes before a session, but sometimes you'll want to go a little further.
You could take your notes and brainstorm other possibilities based on what you do know.
You could do a mindmap and get a visual on how the clues gather together and where they seem to be flowing (excellent in a political or conspiracy type game).
You could draw up a relationship chart of the NPCs and pencil in who dislikes or likes the others.
You could use the facts you've already gathered when talking to witnesses, suspects, or other NPCs, either to double check their details or catch them in a lie. If you want to use it this way, give the Storyteller a copy of your notes to help them keep their story straight and be willing to let them retroactively remove a comment if it was their mistake and not the NPC's slip up.
You could always use it when building your case with the Prince or Primogen, or with the other PCs, to encourage them to take your preferred course of action.
Create a list of leads to follow, or actions to take, and then cross them out as you've done them. This is also a great way to remember the things your character has promised to do.
So there you have it, a whole bunch of uses for those notes you've given.