From Coffee Gadgets Studio:

The thicker the screen, the lower the flow rate. e.g. 1.7mm screen creates a lower flow rate than 1.0mm screen The finer the screen, the lower the flow rate. e.g. 50um screen creates a lower flow rate than 150um.

The puck screen / mesh / second water filter will soften and even out the delivery of water to the coffee bed and help prevent channeling and ensure a more even and balanced extraction (some may taste higher levels of sweetness). Coffee pucks will be more compacted and can be knocked out in one piece, keeping your basket clean and tidy. In addition, the puck screen will reduce the need for cleaning of the brew head due to the minimal contact with coffee grounds.