The recommendation allows you to know the maximum amount f paint the sprayer is capable of working with in a year. If you plan to do work for other people using this sprayer and you plan to use more than 125 gallons of paint per year, then you will need to purchase a different sprayer that is designed to handle a heavier work load.
If you are a homeowner, and will not be using the sprayer to spray 125 gallons of paint per year, that is okay. However, Graco does make a full line of sprayers that are smaller and may be more suitable if you only plan to use it for a couple of gallons of paint per year.