Water quality as in nature is highly important for the animals as well as the plants in an aquascape. Tap water has been known to contain problematic substances like heavy metals (e.g. copper) in too large a quantity, or may be treated with chlorine. Fish and shrimps will not do well in such a water, and many aquatic plants will grow poorly under these conditions.
In these cases, a water conditioner may help as it binds heavy metals in the water, thus rendering them harmless. In addition, a water conditioner removes chlorine and creates a favourable environment for fish and shrimps.