Calcium presence in water creates 'hard water', which you typically don't want when you shower, however you want calcium hardness in your pool to prevent the water from leeching minerals from your pool itself.
Soft water becomes corrosive (we want HARD water) and wants to take minerals from the metal, grout, tiles, stone, concrete, plaster. (metal corrosion could be the pump motor, pipes, rebar structure of your pool). Signs to look out for: pitting of concrete pool flooring and wall; corrosion of metal parts such as filters, ladders pipes and screws; loss of tile grout and the delaminating of your plaster surfaces.
Have to add calcium and magnesium (recommended level is 150ppm anything less is soft water)
High calcium is bad too, creates a cloudy pool and excess amounts of dissolve particles will cause the water to scale in and around your pool equipment, which can clog and block the flow of water through the pool, damaging your pool equipment