This has been said before, both here and in other places, but I wanted to take a moment to go a bit more indepth into how a character sheet is a method for a player to communicate about the game and what they want to do in it. This may be a bit shorter than my normal posts, but well, I'm sure no one is complaining.
Getting down to it though, a character sheet is a powerful tool for GM/Player communication, it is raw mechanics yes, but in systems with point buy mechanics, where a player has points and chooses what it is they want those mechanics tell you a lot about the character, and by that what the player wants to do with the game.
For instance, if a player turns in a character with a lot of social stats, skills, and abilities, then clearly they want to be able to talk their way out of problems. They want to be able to make people trust them, use charm and grace, social finesse, to work their way through things. More than that though, it means that they want Guards that can think, and a world where the opposition are not just brainless automatons. How do you know this? Have you ever tried to convince an automaton to do anything it wasn't specifically told/programmed to do? It doesn't work. The same is true with the 'thinking' guards. While dumb guards may be possible to lie through, that is about the same as constantly giving a combat-cracked character level 1 nobodies to fight. It will get old, and the character is never really challenged at all. We also know, this character is going more for finesse, as said social is more 'finesse' than brute force like a combat character, so we can assume the player wants the chance to think, be cagey, and perhaps run some plots of their own. All this, from just where the player put their points.
That isn't all however, the sheet can tell us more. Looking to disadvantages/flaws/whatever your system calls them we get a list of the character's weaknesses. These are more than just "free points", or at least I think they are. These are the player telling you when and how he wants to be challenged. A character's weakness defines its strength, and that strength doesn't get defined if the character never comes face to face with it. If a character has a phobia of horses, they want to have to deal with horses at some point in time. A lost love? They want to have to deal with those memories and how the love was lost. The character won't kill? They want to be put into situations where they may have to kill, where killing may be the only way to get what they want, and they have to choose between the two.
That is just the beginning really, and if you really look at a character sheet they can give you much more detailed information about both of those things. All a character's skills can tell you how the character plans on dealing with things, where flaws are, or come from, can tell you more on how they want to be challenged.
So, as a player, when you take disadvantages or assign points, ask what you are telling your GM that you want to do in the game. Don't believe in the concept of 'free points' either when taking disadvantages. If you take it, you are telling the GM you want it to come up, and at a dramatically appropriate moment (read: with the worst timing ever! ;) ).
As a GM, when you are looking over a character sheet and adding up points, also ask yourself what you are being told about the character. Does he/she want more combat? Less? Confronted by their past? Or just dealing with insecurities or self-inflicted restrictions? The character sheet is a powerful communication tool, I highly suggest finding how to utilize it to its maximum effectiveness for your game.