This stage is not always helpful for every genre but it can certainly help to have your drums punch just a little more aggressively. This method of compression utilizes the high pass input filter to detect only the clicks and transients of the drums. When reacting to such a quick piece of information, the compressor will dig into the sound slightly after, creating a brief, short duck that increases dynamic range. This slight dynamic range adjustment creates a harder hitting impact on the front of the drums for more punch and movement.
With a compressor designed to catch transient information such as this, paired with the compressor from the mid/side compression stage prior, accurate transient and body control can be maintained far more effectively based on desired outcome.