I'd like to say it saves me money, and it did to start with but then I bought two fancy (relatively speaking) bikes.
So my main driver is to avoid the tedious car commute. The trade off is a slightly longer commute on some days, but when traffic gets bad, and it often does, I save a LOT of time. However, cycling kills many birds with one stone. Cheaper than car, better for environment, no need for boring cardio at gym, fun, simple, liberating, healthy.
Bicycles are great social equaliser and make transport easier for the masses. I only wish more infrastructure was dedicated to them.