Training Tires or racing tires
The difference in rolling resistance is small but can be seen in time trial results and maybe in mass start races. The more noticeable difference is that typically race tires have smoother ride because the thinner casing transmits less vibration.

Note that the categories are not clear and one manufacturer's tire marketed as training tire may have less rolling resistance than other's fastest race tire. One web site to check is http://www.bicyclerollingresistance.com/. 

Is there a necessity? If you are racing and it is necessary to do everything possible to improve your placement.