Well, it's not exactly a novel idea - it's used in a lot of RPGs. Your character levels up and now you don't learn as much from the easy guys you were fighting before. You have more power and now you need to go take on tougher challenges. People like this setup because it's fun that your character progresses and slowly changes, and also fun that you continue to be appropriately challenged.
Now about the difference between basing it off of earned karma vs spent karma: If it's based off of earned karma, it works pretty much as described above, with one potentially adverse side effect on players: you are forced to make use of the extra power granted to you or the enemies will start to be too hard or give you too little karma. Of course, you can argue that having an incentive to make use of your new powers is a positive thing, too - it encourages you to try out new abilities.
If you base it off of spent karma on the other hand, there are a couple weird incentives. First, there is a general incentive to stop at a low total of spent karma for a long time, and just maintain a static character until you've reached the amount of karma you were shooting for. It's like if in a computer RPG you could just farm the tutorial enemies for 10 hours and then skip the end of the game - that's no fun, because it's supposed to be about the journey, not just about reaching the destination. In my view, your shadowrunner should get to experience a variety of circumstances and missions over the course of his or her career. Second, it creates some weird thresholds that you are encouraged to game. For example, look at some of the 10 karma and 15 karma stickers: they are actually fairly similar in power. But if you get penalized for spent karma, the 15 karma ones are WAY more debilitating to put on your character than the 10 karma ones.
Hope that explains it somewhat.
