The main benefit is lower lag time and less stressed yeast. Stressed yeast leads to off flavors, and a long lag time can allow infection to take hold. In a lower gravity ale, one pitchable yeast package is probably enough. Still, a starter never hurts and gives you a "leg up" on getting fermentation going.
According to How to Brew by John Palmer, the pitching rate also affects the aroma/character of the beer. Lower pitching rates tend to produce more aromatics and esters than higher pitching rates. I'm not a chemist, but there are also charts for pitching rates in there, too. Really, the basic idea is that yeast first reproduce as much as needed, then get to work on your beer. So, if you make a starter and they reproduce some there, less of that needs to be done when pitched so fermentation can begin.
I almost always make a starter when I used liquid yeast but have had a spur of the moment brew session where I didn't. Still made good beer but I like the insurance a starter provides.