As great as cities are, there are many issues, many of which generated by transport. Cycling offers a unique solution to solve many of the expected and unexpected problems.
Traffic in some cities is getting to an unmanageable point. We all know that single occupancy vehicles are not an efficient way to allow people to move around. If in section of grid lock traffic the people were all to ride their bikes, the flow would not only be constant but well under the maximum capacity. There are many cycling infrastructure projects on the way across countless to increase cycle ways, and this should be a serious contribution to solving the traffic issue.
In a major city, a road was converted to bus and cycle only and there were lots of complaints that local commerce would suffer. The theory being that less people would stop and spend money in shops and restaurants. It turns out this theory is false, commercial activity increased. This largely due to greater throughput of people (bus and cycle) but also greater opportunity and ease of parking up a bicycle. If you think about it a parked car denies the space for others to park, less parking equals less customers.
Pollution generated by vehicles is outright dangerous, thousands upon thousands of people have their lives shortened by it. We are not only talking gaseous emissions but also particulates emitted from exhausts and brakes. While electric vehicles do solve this problem to a degree, there are many problems with vehicles in cities as mentioned above. The current growth rate of cities makes vehicle usage unsustainable due to the existing constraints. Which places ever more importance on solutions such as cycling for not only our health but the health of all.