For a newbie assembling a bike might not be the best option, but after you know what you like, IMHO, it's the way to go. My first bike was a Bianchi Volpe and I used it to learn about bikes and what set ups suited me. After that I changed a few things on it and now I'm on my fifth bike build.

I build my bikes mostly as I like to have top quality components and those don't come standard on most bikes. I tend to buy 2 or 3 year old components so that I can get the best quality at good prices in places where it matters like hubs. I also have peculiar likes regarding gearing. Building also gives me an intimate knowledge of my bikes so I can handle any roadside fixes. Knowing how to remove your cassette so you can replace a drive side spoke is useful, although on the road I'd use a Fiberflex spoke. I also build just for the fun of it and so that I can ride something a bit out of the ordinary at a good price. My latest build is a De Rosa Neo Primato road bike and obviously it's a bit esoteric as it's an Italian steel frame and uses components you wouldn't generally see on a touring bike, but I've saved $1000 on what I would have paid for a similarly equipped bike from my LBS. I also saved $1000 on my Rambouillet touring bike as I got $500 off the frame as it had a couple of chips in the paint and then bought my components carefully looking for deals on Ultegra level parts.

http://wheelsofchance.org/tag/neo-primato/