Bicycling is a great way to get fit for a number of reasons.
Cycling is a low impact excercise, causing less strain and injuries than some other fitness exercises. It's easy on the joints.
Cycling is easy. Unlike some other sports, cycling does not require a high level of physical skill. When people talk about something that's easy to do, they say "it's as easy as riding a bike." Lance Armstrong got into cycling because he wasn't any good at football at his Texas high school.
Cycling is good for strength and stamina.
Cycling is as intense as you want it to be. If you need to take it easy, doing so is very easy on a bike. If you want to get to 80% of VO2Max and get the heart really pumping, the bike is still the tool for the job.
Cycling is fun. Being outdoors and having fun means you're much more likely to stick with cycling as a fitnes program.
Cycling is social. Group rides for a variety of skill, interest, and fitness levels are available in many areas. If you don't like to be in groups, however, cycling is still a great activity for the loner.
Whatever your motivation for physical fitness -- whether you're training for a summer charity ride or you want a better quality of life -- cycling is a great way to become physically fit.