It would be great if you could just fill the pool with water and not have to worry about algae or bacteria growth. But the reality is that you need to add chemicals to the water to keep nasty contaminants at bay and make the pool environment a safe space for one and all.
Every time you swim, you leave behind body oils, traces of perfumes, shampoos, soaps, and other products in the pool. And there are the twigs, leaves, and debris that fly into the water, making the pool a solid breeding ground for all kinds of micro-organisms.
You will need to use sanitizers and oxidizers to disinfect the pool, and add chemicals to balance the water attributes: pH, calcium, and chlorine levels. You may want to add unique products like algaecides.
However, you need to remember that you must not mix certain chemicals directly, as the combinations may have the potential to injure people.