RO & UV are a technology that is used to purify water. Both significances are different in the point of view application. RO means reverse osmosis removes dissolved organic or inorganic contaminants from the water. UV means ultraviolet is used to kill microorganisms in the water.
Present bacteria, fungi & viruses in the drinking water are very harmful to our bodies. The UV light kills 99.99% of bacteria, fungi & viruses in the water & makes it healthy to drink.
So both are important with different aspects. If water TDS is very high, then we need RO +UV technology-based water purifier to remove dissolved contaminants & microorganisms.
If the water TDS level is under control, only we need to control microorganisms, then the UV water purifier is based.
You can gained detailed knowledge about this topic from the post Water Purifier Buying Guide Step by Step.