I am using RO filter for last many years. One need not use RO filter just for the sake of it. Water filters generally do various stage filtration and purification by removing the sediments,and floating impurities by suitable filters . Some of them have the UV sterilization using Ultra Violet lamp which sterilizes the filtered water and makes it free of much of the harmful organisms.
However in many places only hard water is available for use. Water becomes hard due to the high amount of various salts dissolved in it. One example is sea water. Bore-well water in places where rains are insufficient and near coastal areas are hard. High level of dissolved salts make water taste different and not likable,makes cooking long and may discolour food cooked with that, it may consume more fuel as hard water boils at a higher point. Hard water leaves the vessels corroded and have layers deposited in them.
RO filters reduce the salt content and make the hard water soft and tasty to drink and suitable to cooking . A good RO water filter will have all these stages of a normal water filter and additionally the RO filter or reverse osmosis treatment. RO treatment enables removal of dissolved impurities of salts and harmful chemicals also. It uses a semi permeable membrane through which the water is pressure pushed by the motor installed inside. The water that comes out of the RO membrane will have very less dissolved salt as they are filtered by the semi permeable membrane. RO filters also does not make the dissolved salts totally nil and they cannot make selective filtration.
Nowadays RO water filters come with TDS(total dissolved salt)controllers also.
Regarding a very relevant question posed .."companies claim that their filters are adding minerals to reverse osmosis water at the final stage of filtration. Is it possible to add similar minerals, which were there before filtration? "
I have to say that there is no provision to selectively add the slats or minerals. The TDS control is made by adjusting the flow of RO treated water water. The TDS is mesured by the electrical conductivity caused by the slats in the water.
Water with very low or NIL TDS is also not god for health.Body will not get the needed minerals. It will also taste bitter.
So a permissible level of TDS is to maintained. Usually the municipal water TDS is kept around 50 ppm. The how RO water filter should also keep it similar. So if the TDS is very low, the TDS control valve is adjusted so that the RO untreated(but other wise filtered ) water is allowed to mix with the RO filtered water maintain TDS to an acceptable level.
"Is RO water purifier good for health? Are there any disadvantages of RO water purifier?"-
Hygienic water is good for health. If you are getting treated municipal water supply then, RO filter is not necessary. If you are using your own well water with soft water then also RO filter is not necessary.In the rainy seasons or when in summer the water pressure is not good or water gets contaminated, normal water filter with UV is enough and you can take extra precaution by boiling the water.
However when you don't get treated municipal water, and depend on the hard well water or bore well water then you may need RO filter. RO should be used in places where the input water has TDS more than 100.
You may keep the out put around 50 . Whenever the water starts tasting different you may have to adjust the TDS or check for any fault.