Water plus/Maximum Water level, extra rinses, Sensitive, etc... in some form or another are offered by most European front loaders along with others.
Basically they are meant to allow certain modifications of water levels in wash and or rinse cycles, or adding extra rinse(s) to cope with various wants of a particular customer within limits.
Since these machines are sold all over Europe, North America and elsewhere they must cope with water quality (hardness) that cannot always accurately be predicted.
Water in many areas of USA (such as New York) tends to be far softer than what you'd see in many parts of EU. Soft water can complicate rinsing. Ditto if a home or wherever machine is used has a mechanical water softening system installed.
Miele or others also cannot account for detergents and other products used on wash day. Some things rinse more easily than others. This and or consumer may be prone to using more than they should. Certain households may have one or more members prone to sensitivity regarding detergent or other laundry product residue, thus want things rinsed more.
At least for Miele Water + seems to be for coping with loads that absorb lots of water.
Modern computer controlled washers will only use water levels as programmed within certain parameters. Someone mentioned their washer having so little water during a cycle the recirculating pump was running "dry". Well if washer is programed to give "X" liters of water max for "Y" pounds or kilos of wash, that is what happens. Water Plus moves that needle for two more preset levels in aid of solving that problem.
With machines of old one could simply pour a kettle or so of water down dispenser to raise water levels. Nearly all modern washers have various water protection systems. If levels in tub go above what is programmed machine senses this, believes something is very wrong, aborts cycle and drains water. You can still try that method, but there is no way of knowing where "sweet spot" lies before triggering WPS.
Manual for my older Miele suggests washing less than full loads when doing very absorbent loads such as terry toweling.