It removes the plasticizers in the line's coating, the chemical which makes plastics soft and flexible. Once removed in sufficient quantity, the line becomes stiff and brittle and line's coating will begin to crack. Line manufacturers are working with the producers of mucilin products to inform folks not to use mucilin on modern fly lines. Most folks thinks it's just dirt. It's plasticizer evaporated from you car's vinyl interior.
Dashes crack and so do seat covers.
One of the worst places to store or transport your lines is in the trunk of your car. Don't purchase a fly line that has been on a shelf in front of a window, it may be already damaged from heat and UV light. Avoid contacting your lines with solvents, gasoline, sunscreens and insect repellents. All will damage your line's coating by removing plasticizers and other needed chemicals. Some of these will dissolve the lines coating.