Natural cold processed soap is made up from plant based oils and butters. No artificial or synthetic ingredients, therefore what's being washed down your plug hole is natural.
For every synthetic soap you use, the environment goes down the drain too. The chemicals used in commercial soaps not only damage your skin (and health) but there's also damage to the environment and aquatic system when you wash it down the drain during a shower or bath.
If you can imagine millions of people using soap every day and these harmful chemicals are being washed into our waterways and end up in the aquatic system where they harm marine life.
There is also a smaller carbon footprint if you buy your soap from a local soapmaker. Using local ingredients, local labor and making small quantities helps to reduce the carbon footprint of the locally produced soaps. There isn’t as much requirement of fuel, transport, freight or other resources which can harm the environment.
The drains in your house (showers, toilets, sinks, etc) drain to the sewer. The sewer then carries your soapy water to a treatment plant. At the plant, it's then cleaned through filters and chemical processes and depending on where you live, it's usually released to the environment. Even more reason to cleanse with something that isn't going to give the lakes, rivers, oceans a hard time chemically wise.
Our skin is like the environment, we need to look after it with love and care. This includes checking out the ingredients on your soap and if there's chemicals (or palm oil), give them a wide berth. There's a good few of us natural soap makers out there, the best compliment is seeing people read the ingredients and finding nothing that will upset our skin - or the environment.
It's the same logic as a plastic straw. Why does one plastic straw matter? It could be nothing but when you imagine it in millions, it sure makes a massive difference to the environment.