It depends on what area you are in, but in most places, if you have tap water, municipalities add additives which can be potentially harmful to reef animals. This can include compounds such as chlorine, chloramarine, phosphates, nitrates, trace metals, etc. RO is short for reverse osmosis, DI short for deionization. The multiple step process forces water through a membrane and other steps which bind everything except pure H2O, taking them out of the equation, so that when you add salt, you are making the purest product possible.
The quality of your water will differ depending onwhich area you live in, and there are also people who depend on well water (which can still have impurities from the water table, depending on geography, but will have fewer artificial additives), so YMMV (your mileage may vary). Check with your water company if you have questions about how your water tests out.