Some people have cleaner tap water than others. Different municipalities use different sanitation methods, different disinfectants, have different geology (aquifer location and mineral contents) and ultimately what comes out the tap varies. Not all cities/towns use chloramines (though many do) and not all add fluoride to the water.
If you are really interested in exactly what is in your tap water, you should check out your municipal water report. In the USA (other countries as well I assume) your city/town or water company should mail you such a report annually. It is also likely to be found online.
Fluoride, chloramines and chlorination by-products aside; tap water contains minerals like calcium, magnesium, iron and copper which the plant can make use of. RO water is not necessary. Unless your water is really bad, if you wanted to filter it you could use an activated charcoal/ion exchange resin system (Brita/Pur) or a multistage filter. RO systems waste a lot of water.