power supply, rails

I am going to be building a computer this coming summer and I was doing some preliminary research on power supplies. I've read the cons for having multiple rails (amperage limit on each rail, so if you have something that demands more amps than a single rail can handle, then you won't be able to run that. Also if whatever you run on that rail uses less than the amperage limit, then you waste the remaining amperage capability), but I haven't heard what the positives to having multiple rails are.

This is the PSU I've picked out:

http://www.pcpower.com/power-supply/silencer-610-eps12v.html

It's a Silencer 610 EPS12V

Oh, and this is my first build.

 

I'm no power supply expert, but I think one of the plusses of multiple rails is that if you have several devices pulling serious amps, they don't pull all the amps from one rail and risk overworking and taxing the single rail. With several rails, each high draw device can have it's own 'dedicated' amperage and voltage, leading to smoother and more stable operation. I wouldn't worry about 'wasted' amperage (honestly, the concept of wasted amperage doesn't even make sense to me). As to whether a device will draw more than one rail can supply: just check the requirements of your devices, and then buy an appropriate power supply. IMO, over 28a on a single rail and over 18a per rail on multiple rail PSUs is fine. Your selected PSU has 49a on a single rail which should be plenty.