Explain Like I'm Five

Blue is the B in RGB. It’s an essential wavelength used by RBG or RGBW (which includes white pixels) displays in order to accurately reproduce the various colors we want to look at. They are carefully blended together to reproduce billions of different colors.

Blue light has a higher frequency than red or green light, so prolonged exposure can be fatiguing on the eyes. But this usually requires many, many hours of screen time over many days - which I suppose is quite common for a lot of people. There are other factors which arguably contribute more to eye strain though such as uneven backlight strobing which can be an issue for lower quality displays.

The reason you can’t just turn the blue light off is you wouldn’t be able to accurately produce a ton of colors. Even if you aren’t necessarily viewing something with a ton of blue color in it at the time, removing all blue light from the equation would alter most of the other colors that you ARE looking at. There are software solutions such as f.lux that try to reduce strain by lowering blue light and compensate by raising “gentler” wavelengths, but they produce a visibly warmer, more yellowed effect which can be less than ideal in some scenarios.

What is the natural science reason to wear blue light eyeglasses

There isn't. It's been debunked completely from the get-go. https://www.npr.org/2021/02/21/969886124/do-blue-light-blocking-glasses-really-work

such as by using UV filtering

Your monitor/phone isn't generating UV.

I have a software add on that filters blue out of my screen at night when it could cause sleep disruption. The result is that the screen is overwhelmingly reddish and colours in images are hard to perceive reliably, but if I'm reading that's fine. Other people might not be okay with that.

I don't know what the hubub is about eye damage. That's new to me, but it does trigger the suppression of certain sleep hormones in humans.

As far as I know, there is software that will do this. However, that software usually needs some higher permissions to be able to be able to change the colors of the computer's whole output. For windows this is usually not a problem unless it's a work PC / laptop. For Android it means you need to have a rooted phone, otherwise no-blue-light apps will just apply a red filter on top, but not actually stop emitting blue light.

If you've got a rooted android you might want to try to flux app.

https://play.google.com/store/apps/details?id=com.justgetflux.flux

I suppose my explanation is out of scope, but that's the only real reason that I know of.

It is my understanding that most LEDs are inherently blue, but they add phosphors that mask the appearance of that blue light, making it appear white to our eyes.

If you look at the spectrum for an LED bulb there is almost always a big spike in the blue part of the wavelength.

So, if you were to "turn off" the blue light, you would be turning off the source of the light itself. Because of that, filtering is often used to lessen that blue bump in the spectrum.

The reason is that the assumption 'no other light sources aside from the screen' is almost impossible to control for. You'd have to be in a room with no daylight coming in and zero other blue light sources emitting from say a smartphone, clock readout, artificial lighting for the room and/or any of its appliances.

Even accepting this ridiculous decision to rule 'all other sources of light out of scope' then the obvious only possible answer is that THE SCREEN EMITS BLUE LIGHT OUTSIDE THE CONTROL OF SOFTWARE COLOUR SETTINGS. For example, LCD screens use a white light bulb to light up a screen of LCD pixels. This white light leaks. It's why LCD screens have lower contrast (black looks dark grey).

I'll tell some background on why I asked the original question.

I am using a setup where I have adjusted the amount of blue light emitted by my display to as low levels as I could from the interface accessible through the physical buttons in my display. My display is also set to be so dim that when some other people took a look at my display, they had a hard time seeing almost anything even at a good viewing angle.

I also have the night light feature set to as warm as possible on my Ubuntu Linux.

I can also easily control all other possible light sources in the place I am in, including blocking virtually any and all sunlight.

I do know that ambient lighting can be beneficial to the eyes, however. I solely came up with the original question out of curiosity and actual real life setting in mind.

The display I use does look like it is from the control panel of a submarine but that is not a problem to me as a programmer that uses no syntax highlighting as I see no need (or any positive effect from highlighting in my case: I wouldn't read a novel with different words in different colors; I find such features in editors distracting (and Vim or a similar editor without plugins tends to be pure bliss compared to IDEs)) to highlight syntax while programming; all the semantic information I need is in the source code text (i.e., without color data) itself (excluding e.g. colorForth when rendered the way that language is (AFAIK) typically rendered on the screen).