Speed sensors help in 2 ways. 1) They reduce tracking errors inherent in GPS receivers (dedicated and phones) when using in areas with a lot of tree coverage (or steep walled canyons such as Manhattan). Thus and if mt. biking or gravel road riding in a national forest, as example, having a speed sensor improves distance and speed accuracy. 2) GPS alone tends to lag in displaying actual speed AT THAT MOMENT. The only time I've found this lag to be annoying is when doing fast group rides where I'm trying to maintain a steady speed and using the computer to verify that speed.