So that people could... 

As a cyclist living in Park Slope, I can't ride down 5th avenue without swerving around a parked car in the bike lane and into traffic. As biking is a contentious issue, 30 minute parking spaces would hopefully discourage people from parking in the bike lane which would alleviate the stress and danger for both motorists and cyclists. Also, hopefully with people using actual parking spaces instead of double parking all the time, streets would be clearer and motorists would not constantly have to slow down to squeeze past double parked cars or lay on their horn until someone came to move the offending vehicle.