Wool sweaters do not need to be washed after each wear. If some care is taken, washing can be avoided. For example, wear a light t-shirt under your sweater to protect it from body odors or hang it outside for 15-30 minutes to freshen it up from smells of food or smoke. Spot cleaning can be great too!
For woolens in particular, no rinse is an important concept. Wool fibre has barbs on it that have a tendency to link together when agitated or washed in the wrong temperature of water. This leads to "felting" and shrinking. Using a no rinse product ensures that the water temperature is not being changed between the wash and rinse water. It also means that you are handling the fibres less and decreasing the danger of "felting" it. The lanolin in Eucalan is a natural conditioner for wool, so it helps to increase the loft of the fibre and decrease the itchiness of wool by relaxing the barbs.