If the car in front of you starts getting the air moving by driving through it, it takes less energy for you to have to push the air so that little extra energy turns into a little extra speed. If you watch motorcycle racing or car racing they do it all the time. Sometime 5-15 mph difference..

Less resistance means less work you car has to do. The physics are the same on racetracks or roses and the distances between cars where a slip stream works are greater the faster you go so it usually isn’t worth the risk at high way or normal road speeds..