If you mean the setting the amps to 6, 7, 8, 9, etc.. Amps then it's because some places may trip if running at max amp's for that type of socket.My further question is..... what is the purpose of the "charging rate" adjustment on the display? if the car determines the charging rate then should it just not be left on max all the time?
For example you have a old cottage in the hills of Wales. You have a 60A supply and even the kettle can trip your main breaker if you put it on at the same time as the Oven. Setting your charge at a low ampage means it can still charge without tripping the whole house.
There are a few other reasons but hopefully you get the idea.