Improve speed under bridges, tunnels, and trees. And speed on a trainer.

The comment above about buildings/trees/etc is true, but also remember that GPS is only accurate to a certain degree. The speed sensor is like the next step in that calibration. Honestly for cycling it never matters to me. If I'm +/- .1 who cares. Same with my mph (or kph for you people who do it right)- if im up or down a bit I just don't really care, but there are lots of folks who do. In my custom screen I ride the most with I actually don't even have speed on there- just time stuff and wattage

thanx for the feedback.

Also, you may have a dumb trainer and want to have a speed sensor for indoor training as well.

If you use the chung method for aero testing, they typically suggest using a speed sensor over gps for consistency in numbers as well.