D&D, Pathfinder and similar rulesets all have a system whereby your characters grow in power by accumulating enough experience to level up. The game is also designed so that challenges the DM places in front of you to overcome are balanced for your level by taking pains to ensure each character's wealth is roughly on par with their level.
But sometimes, an award of additional XP seems warranted. Maybe successfully completing an adventure or even a chapter of it doesn't have any kind of wealth award attached to it, but the characters definitely "learned something," and a mechanical recognition of their endeavors is appropriate. Each XP awarded in this way further reinforces the idea that the game is not all about combat. Even side quests might net the PCs additional XP, and it could also be possible that completing an objective in a particularly efficient or heroic way is worth bonus XP above and beyond what was already slated; killing a necromancer before he can animate the corpses of his victims precludes the possibility of also fighting zombies or skeletons, for example, but arriving in time to defeat the villain before he kills those people to begin with FEELS better, even if the fight turned out exactly the same way.
There are some problems with bonus XP, though. If you don't keep up with the wealth by levl guidelines, obviously you are going to run into an issue at some point down the road. Around 4th or 5th level, for example, PCs are expected to have one or more magic weapons in their collective arsenal; without it, incorporeal creatures or really anything with damage reduction will be several magnitudes more difficult to overcome. And eventually, even if the party's cleric can cast raise dead, he or she can't afford to without 5,000gp worth of material components. It would suck to lose a character through no fault of your own but only because the DM is a cheapskate.
Also, there is some dispute about whether it is appropriate for the various characters in the party to be of different levels. The rules are pretty good about allowing folks to catch up, sort of, but it can be kind of a drag for individual PCs to always be lagging behind. And it is also true that maybe some characters, either by virtue of their abilities or the way they are played, are more likely to find themselves in situations where they can rise above the pack and enjoy their time in the spotlight. Again, this isn't anyone's fault but the DM's for not allowing other folks the chance to soak up some of that precious extra XP. On this latter point, I am of the opinion that if the DM is mindful of this phenomenon at least the possibility of other characters catching up will come up over time.
What are everyone's thoughts on this? In a beer and pretzels, kick-in-the-door kind of campaign where people just bulldoze through dungeons to kill monsters and take their stuff, this doeen't come up much. But it seems like campaigns with a lot of dialogue, character development and side quests are more prone to what might eventually be unbalanced and disruptive.