There are guides to awarding class experience in the 2e DMG. That table has been reproduced below for your perusal. This concept can be taken and adapted to many systems that give out generic experience or other types of character-advancement points. The lesson of class experience rewards has already been incorporated by systems like Hârn, BRP, Aftermath!, and CoC where abilities improve when you use them as you use them.
What, then, is that lesson? These awards serve a number of purposes. The first is to reinforce class archetypes. The second is to guide play. The third is to simulate an in-game effect. Each of these purposes is important, though I would argue that reinforcing class archetypes is perhaps the strongest of these factors while guiding play is the weakest.
Reinforcing class archetypes. Warriors war and wizards wiz, and this is an essential element of the playstyle of earlier Dungeons and Dragons games. The obsession with mixing and matching skills and powers to create nebulous classes or to deconstruct classes altogether does not pervade pre-Skills and Powers D&D (note that I actually like a lot of the Players Option books... just not the character building of S&P).
There are good in-setting reasons for these archetypes that are built into the core assumptions of D&D. We aren't, for example, considering an underlying setting in which anyone who says the magic words can cause magic to function—magic takes dedication, both mental and physical, and an understanding of deeper philosophies. These class archetypes (warrior, wizard, thief, etc.) dictate the career of a character and the things they will learn. However, absent the rules awarding those characters with experience for doing class-like things, there is no other incentive to adhere to the archetype.
The presence of individual class experience encourages players to do class-like things. Wizards gain experience for using magic to solve problems, which encourages them to apply spells to as many problems as possible. Warriors get experience for fighting. When all you have is a hammer, everything begins to look like a nail. This works the players into the player-character mindset, reinforcing the tools those characters have at their disposal to approach problems.
Note that I generally give ALL classes xp awards for diplomacy, thereby making the diplomatic option equally attractive to all players. This economics-of-experience can mould and shape the way players get into character, which is pretty cool.
Guiding play. In much the same way as the xp awards reinforce the class archetypes, they also shape the play of players. Whereas class archetypes deal with in-setting reality, play-guiding deals with out-of-character incentive. These two work in hand in hand; above and beyond the effect of reinforcing archetypes, however, the play-guiding effect teaches players how to play that type of character.
Simulating in-game effects. The in-game effects are alluded to above; wizards learn how to use magic better by using magic. Warriors improve their skills at combat by fighting. While it is potentially possible for a character to go from level one to level twenty without ever getting into a combat (although I can't imagine the insane quest rewards the DM must be giving out for this fabricated character), it makes little sense for a warrior's ability to swing a sword to improve simply because he talked to a bunch of people and worked out a problem.
Let's examine a low level warrior killing, say, goblins. He is likely in a party; for sake of ease, let's say a party of 5 people. Goblins are worth 15 experience points each. Because he is with 4 other characters, he will get a total of 3xp per goblin on the straight reward value. But, the individual award for a warrior when fighting a goblin is 10xp per. This is three times again as many experience, just for him. Because fighting is what he does, improving his fighting happens faster when he is actually fighting.
In a scenario with 10 goblins, the warrior will receive 30xp from the goblins and 100xp from individual class awards. A wizard, however, will receive the flat 30xp. If the wizard used a level one spell throughout the course of the fight, he will get an extra 50xp, for a total of 80xp. HOWEVER, spells have more utility than simply combat: that wizard may get an extra 50xp next time the party goes to a town because he used a sleep spell to knock out some knights and help steal their manor-keys.
For these three reasons, I love individual class awards.