Why should I add MB Windshield Washer Fluid?

What is special in MB windshield washing system?
Why not ommercially available premixed windshield washer?

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In the manual (1992 300D):

MB Windshield Washer Concentrate "S" and commercially available premixed windshield washer solvent/antifreeze

1 part "S" to 100 parts solvent
(40ml "S" to 1 gallon solvent)