Maybe because the whole "blue light in displays" has no real effect on our sleep

That is a blue light filter, so you shouldn't need a third party filter. The science just isn't out there to show whether or not it helps with sleep, though. It might help, but other things like not using screens within a couple hours to bed and not using phones in bed has better outcomes.

I can't say I notice any difference, either**, but it does seem like it reduces headaches for me and makes it easier on my eyes in low light.

** I'm an insomniac, so "better sleep" is often "any sleep I can get" lol.

If you really want better sleep quality probably a better way is stop using stuff with screens like an hour or so before bed. It's not really the blue light that is the issue, it's the endless content keeping our mind active.

Give your brain time to rest and relax!