VEX IQ Smart Motors can be programmed to spin or drive at many different velocities. If you have programmed your robot to drive or turn before, you probably did it with the default velocity. The default velocity is the velocity that the Smart Motors turn at automatically - if you don’t add any type of block for setting the velocity. The default velocity is 50% of the maximum velocity. The maximum velocity is 100%.
The blocks for setting velocity are important because you might want your robot to drive or turn faster or slower. For example, you might be racing against another robot and want your robot to drive as fast as possible. Or, you might want your robot to drive or turn more slowly like when it has to carefully complete a task like moving through a maze.