Totally not a stupid question! My understanding (and it's a very limited understanding, so take it with many grains of salt), is that the benefit of juicing (or one of the benefits) is that the nutrients and sugars are more immediately available to your system via your bloodstream, so your digestion can take a little break. If you are doing a juice fast (or juice "feast"), or if you juice in the morning as your system is waking up, it's a great cleanser.
For me, the immediate sugar rush of juice is too much, and I like the fiber and the weight of smoothies. But that's just me and the way my body works best. I know plenty of folks who swear by juicing. Also, my kids love doing green juice; although they will drink smoothies, they won't drink a lot of them. They will, however, drink gallons of "green lemonade" or "green apple juice" - which I make by juicing (via the blender).
cheers - amy