Research is inconclusive, but there are indications that UV light (although not required for photosynthesis) can have effects on yields and either on resistance

Research is inconclusive, but there are indications that UV light (although not required for photosynthesis) can have effects on yields and either on resistance against pests (insects/molds/fungi) or directly attack/kill/weaken those pests. So basically 'it depends', and people do their own tests and then determine if, in their specific circumstances, it is a net positive (economically) to add UV lights to their setups. I think this is the main message. People in this thread are talking in absolutes about 'agriculture', but the reality is that there are many, many things that we don't understand well enough at the above-micro-level scale to be able to say 'you need to measure x, y and z and then we can calculate/model the optimal setup to grow something'. People are basically relying on (informed) trial and error, with feedback cycles measures in months or years, so it just takes a long long time for progress to be made - and even then, we usually don't know exactly why something works in a specific case. It's not like 'oh let's run an A/B test for the best color of our 'buy' button for a few days'.