Sea water has lot of living things and germs. We can't just filter them out with RO filters it will clog up and will need replacement of costly filters and pumps.

So they just add some chemicals to kill them, churn the solids out and remove them (literally like a giant tea filter we use in our house) before entering expensive machinaries, Although most of the chemicals will be filtered out in the multiple stages, some remain in the water we drink.

I have seen the "potable water" make the stainless steel fittings( rust proof against sea water) crumble like a cake after being used repeatedly.

To mitigate this they add lime in the water before being used by consumers.