It addresses several fundamental problems with rolling dice:
1) Players will cheat. With point buy, there is no ability to cheat, which starts the game play going in the right direction from the start. However, this problem to me seems to be somewhat addressed by 3rd edition in general. At least now a character with all 14's is a pretty good character - especially in a class with a broad skill base (ranger, barbarian, rouge, monk, bard). Alot more combinations of stats are functional.
2) It is fair. Why random die rolls, you run the risk of an uber character that becomes the center of attention, or a weak character whose player feels like he can't contribute.
3) It offers the possibility of solid character stats every time.
For all that, it has one huge problem that prevents me from abopting it whole heartedly.
a) It encourages players to min/max and that starts the game off in the wrong direction right from the start. Players tend not to role play or act out thier characters, they account and enumerate them. Plus, it seems to encourage every character to be basically alike. Characters end up with highly standardized packages. You don't see charismatic fighters. You don't see smart barbarians. You hardly see anyone with a 6 or 7 in a stat. Virtually every character seems to start identically as if they were stamped out at the Munchkin factory from a common die.
I've found it works pretty good to give the option of a standard 4d6 take 3 or else let them have 28 point buy.