Because you want the traffic to pass you as a series of individual vehicles, each making their choice about if it safe to pass, rather than as one mass.
When the line of cars is unbroken the potential for the cyclist to me hidden by a vehicle in front is much higher – having gaps between the cars means more opportunity to be seen.
The cyclist asserting themselves in this way is another major benefit, rather than shrinking out of sight and out of mind in the gutter.
All three of those are very important factors. Plus, thinking about this subject really does show how dangerous and what bad driving it is it is for cars to drive too close together.