"why do we need a folding bike?"
- Folded folding bike usually still takes up less space than a full size bike with the wheels off. In my case, the difference between stowing it in a compartment and having to figure out where to fit it into my van's living space. Take the wheels off a folding bike and it stows even smaller.
- Useful for intermodal transportation. It's part of my van-bus-bike commute. Better on buses and trains; less expensive than a full-size bike if you are flying commercial airlines.
- When I go grocery shopping, I don't lock it up out of sight; it comes with me in a grocery cart.
- People like folding bikes. For some reason, you are no longer one of those "scofflaw bicyclists," but instead a smile-inducing novelty.
No one needs a folding bike, plenty could get by with a full-size bike. But there are plenty of situations where a folding bike is a better or the best choice.