UV radiation is a good disinfectant at the cell where the water passes in front of the UV light.
It will also inactivate some organisms that are resistant to chlorine such as Cryptosporidium and Girdia. These cause a diarrhoea type of illness that carriers can retain for weeks. Commercial pools deal with these by the addition of flocculant dosing that allows the sand filters to trap the particulates.
UV cells are an excellent option for domestic pools in this respect. They are not expensive and are easy to install in the piping system in the plant room.
The only drawback to UV water treatment is that the cell only works when the pool pump is running. Bluepools overcomes this by using a small pump and running it 24/7. The extra cost is more than compensated for by a large reduction in the cost of the chlorination chemicals required.
The really large advantage of this approach is that the pool water chlorine content can be reduced to the same level that comes out of the tap.