On straights this bubble gives what is known as a 'tow', slightly reducing the air resistance of the car behind and (all else being even) allowing it a slight performance advantage - hence the reason cars are often seen very close together just before an overtaking attempt.

The problem comes with the second aerodynamic effect, found in corners, where the reduced airflow acting on the wings of the second car will dramatically decrease aerodynamic downforce, and hence grip - meaning that the car behind will typically be forced to drop back, or to pick a different cornering line in 'clean air'.