One advantage of using a DSLR is the ability to change lenses depending on the type of photograph you want to take (see Three DSLR Lenses You Need). Some will argue that lenses are just as important if not more important than the camera body (see Lenses Are the Real Key to Stunning Photos for an introduction to DSLR lenses).
I started out with a kit lens (standard zoom lens, 18-55mm f/3.5-5.6 IS lens) that came with my Canon XSi, but I recently added a faster prime lens (i.e. Canon EF 50mm f/1.8 II) to my gear bag for low light situations. The lens was relatively inexpensive, $99.95 on Adorama.
Being able to change lenses can be a curse in certain ways. First, the vast variety of lenses is a bit overwhelming. Check out the step-by-step guide to finding the right lens for your needs at Digital SLR Guide. Second, the costs of lenses are pretty staggering; many lenses cost as much or more than the actual camera body itself. I can easily see how expensive my hobby can quickly become. Third, instead of just carrying around my camera with the one lens attached, I’m now forced to carry additional lenses in a bag.
Despite all of the inconveniences, it’s no surprise that photographers carry additional lenses with them, especially when you see the beautiful photographs as a result of using the right lens.
I was fortunate enough to visit the California Academy of Sciences in Golden Gate Park just after I got my new 50mm lens. The lens was awesome in the low light setting of the jelly fish aquarium.