well, like you said - it's usually "because i can" - can't think of any functional reason you'd need a $700 reel for a 5 weight - only aesthetic reasons or like some folks (myself included) you collect certain types/brands of reels and want to add to your collection - like most things of a higher price, a big part of it is status - a $20,000 hyundai will get you to work just as effectively as an $80,000 bmw m5 - but there'll be fewer people driving the m5 and a lot more people looking at you when you're driving it - it's a status symbol plane and simple when you're talking that kind of cash for a trout reel - either that or a collector's item.
i assume you've seen some of the beauties sold here - $700 can be a bargain
http://www.just-reels.com/