The TM requirement for daily rinsing specified a minimum standard for the water to be used. There are some places in the UK where the supplied tap water would have met that specification but it needed to be guaranteed, day in, day out. Using de-ionised, demineralized or distilled simply guarantees the quality. As several UK operators have discovered over the last few months, there's a lot more to water than you think!!
PM me if you need more.