Well, the way I see it, as nice as it is to know where you stand with someone, there is something to be said about someone driving in from out of left field and dumping their intentions on me like a pile of cinder blocks. It's friggin' fantastic that you've had time to figure out your feelings about me, but I don't know how I feel about you yet and I'm sure you'd like an answer sooner rather than later since, you know, it probably wasn't easy opening up like that. Unless I've already made up my mind about you, however, being on the receiving end of the direct approach, I'm feeling a bit pressured to come up with an answer.
Flirting kind of allows you to take things for a test drive without actually telling anyone it's a test drive. You give both people a chance to see if there's chemistry, or even build some, so when someone finally works up the nerve to say 'hey, I like you're face, let's mambo,' there's a foundation. The other person can say, 'hey, yeah, I enjoyed interacting with you at that playful-romantic level, let's make it real...' or.... 'errr, I have a better idea, let's not.'
Of course, there are ways flirting can go haywire. Some people are afraid of commitment and use flirting as a way to keep their options open... or they just plain enjoy the sport of it. When they run into one of those 'I'd eventually like to come to a point where we see where we stand' people, that's where the problems start.
Me, I prefer having flirtin come to an apex where things get serious and then directness is needed from at least one party.
I'm horrible at flirting though... 
