A lot of what photography is about is conveying to a viewer a subject and a background. The two should, most of the time, be very distinct. You should not make the viewer 'work' to figure out where one starts and the other ends. Creating separation of subject from background can be done in several ways. In most outdoor on location shots you can get some nice directional/modeled side lighting and maybe even use an on camera fill to bring detail into the shadows of the subject. But often times the shadow side of the subject can/will still be of the same tonality as the background and therefore the subject to background separation is deemed poor. Well the fact is that this shadow side and the background are quite often in reality the same tone. One really cool way to remedy this is to add this kicker, hair light, rim light to the subject so it will create a very thin line of brighter light at the edge of the subject which will now contrast the background and create the separation needed. A snoot helps 'focus' the light to a specific area. Without it you might get too much spill over and light up too much of the subject and maybe even create flares if it hits your lens. Often times if I don't have a snoot handy I will just manually zoom my Canon flash to something like 105mm and keep it relatively close to the subject.
