This setting adjusts the flow rate for the skirt or brim.
Increasing the flow rate of the skirt or brim can make it stick better to the build plate, because the material is pressed onto the build plate with more force. However, care must be taken to not overextrude too much.
If the material sticks out above the brim, the nozzle may bump into the brim and tear it off the build plate when a travelling across the brim.
If the skirt or brim lines overlap too much, some lines will not be able to reach the build plate because they are blocked by overly wide adjacent lines. This can be detrimental to the adhesion to the build plate.
Instead, the Skirt/Brim Line Width can get the same effect of pushing down harder on the build plate, but spaces the lines out further as well so that they can all stick to the build plate.
The flow rate for the skirt or brim can be adjusted separately from the flow rate of the rest of the print.