Well most of the time, its water pumped from the ocean. They put it through filters and under UV sterilizers so ultimately you get water thats from the ocean without the worry of parasites and pollutants.
Mixing the water yourself, you might not (depending on the crystals I think) be adding trace elements to the water, and you would need to regulate the calcium, iodine, and strontium yourself. Because I do 25% water changes every weekend, I dont worry about it because by the time it could get low, most of it gets replaced anyhow.
Plus the water is usually so cheap you dont think twice about it.