Why throw away so much sourdough starter? Tartine Book no. 3
At the beginning, you are throwing out a lot because you are just feeding the culture. You're just seeding the growth medium (fresh flour and water) with the young culture, so you want to be sure that the ratio of food to culture is appropriate.

Once the culture is established, you don't have to throw out any, but you do have to keep feeding it. This generally means that either you throw out a little, or have to feed it a ton. At the bakery where I work, we use so much starter that we don't throw any out. We're left with little enough at the end of the day that we just add fresh flour and water.

The ratio of starter to fresh flour and water (and ratio of flour to water) are used to control the culture. Different strains grow better at different levels of hydration, and leaving more of the already cultured starter in when you feed it can leave in more of the "waste products", like acids, which contribute to flavor.