It can be a way to exercise without taking much times out of ones schedule. The time one would spend biking to work serves two important purposes. One, getting to work, but also as a great form of exercise. Improved mobility in crowded situations. In downtown areas, biking to work may actually save time.
Cars crawl through congested traffic, while bicyclists ride around it.
The time it takes to park a car could also be factored in. Finding a parking space takes time and may be far away, while bikes are easy to lock and can be locked close to any destination. Personal economics are also important. Cars are expensive to own and operate. On top of the high prices for new cars, one must also pay for insurance, fuel, and maintenance.