Games these days seem to keep track of a player's every move, and all sorts of ridiculous numbers, but they don't seem to do much with the data to make the player experience different. This post is inspired by my recent pistols-only playthrough of Deus Ex, which became very frustrating later in the game. The reason is I had spent lots of money and effort upgrading my 10mm pistol with a silencer, higher capacity, faster reload, etc. etc. only to end up with a shortage of ammo during the last third of the game. For one boss fight I had to franticly collect enough ammo to win the fight, during the fight. I recognize that when I'm fighting guys with heavy machine guns and rocket launchers that they should drop ammo for said guns. But random, environmental loot could easily be substituted for the type of ammo I need. It can be done by increasing chances rather than every single loot drop being the right kinds of ammo. Check the players inventory, keep track of number of shots fired from the different kinds of guns, how long held in inventory, how many times it's been upgraded, etc. and you can instantly solve this problem.
This is just one example of leveraging all the stat tracking that goes on to making a game adapt to the way the player is playing. There are many more radical, complicated, and drastic approaches: dynamic level design, dynamic enemy placement, dynamic difficulty (not new, but could be aided by extensive stat collection). You could even imagine at the furthest extreme, a game with dynamic design that invented new game mechanics based on the way players were using the first ones provided.
To reuse the same example, my character focused on using pistols, generally only effective out to medium range. In the dynamic level design case, perhaps you have three different layouts for the same section of a map, optimised for range categories short, medium, and long, swapping them out depending on the weapon most used. For enemy placement you add more enemies, but make them less well armored, and get rid of the patrol robots. All of these ideas are rather crude, the concept would only be satisfying to players if it was implemented in a more subtle and sophisticated manner. RPGs especially are ripe targets for this sort of approach: you have characters stats right in front of you, as well as things like class, race, and inventory. By keeping track of how the player plays the game, when they jump, how often they die, save, reload, find hidden areas, use powers, find powerups, you can make their experience better.