Grapes with too much sugar means a lot of alcohol will be produced and this can cause problems with the primary and secondary fermentations. High alcohol level is becoming inhibitor for most yeast strains and malolactic bacteria and actually kills them, which can cause a sluggish or stuck fermentation. To prevent that some water need to add to lower the sugar levels in grape juice/must. Adding water will not only dilute your sugar concentration, but you will dilute the total acidity of the juice/must as well. For that reason, unless the must/juice already has high acid, it is important to check and adjust your acidity and pH to a proper level. However this practice could cause a dilution in flavors as well, so winemakers need be very carefully how much water to add to their grape must/ juice prior fermentation.