Bike lanes are needed because it helps to create a biking culture in the city, many people don't feel safe so if you build cycling infrastructure ,more people will bike. If you build it they will come. Cars have lanes, pedestrians have side walks, streetcars and buses have dedicated lanes, subways have there own tunnels and bridges, so it just makes sense to create a place for cyclist. Some drivers get angry that people bike on the road, pedestrians get angry when bikers go on the side walk, so the seperate lanes for biking creates a sense of place and helps to establish biking as a credible mode of transportation in the minds of the population.
people coming from cities with lots of seperate bike lanes are use to it and often find it strange coming to a city with none at all, it sends out a message that this city is not bike freindly and that your not welcome.