- Image sharpness and contrast is excellent. You can read text on an old flip display far away that you would struggle to read on a modern LCD if you have mild astigmatism.

- Perfectly readable on a sunny day.

- They can show colored logos and symbols (e.g. train type, airline, highlight warnings)

- Better failure mode than LCD: single digits fail independently from each other and can be replaced. But it's less likely that the entire display fails.

- They are either working or stuck - no fading out like CRTs.

I could just about imagine a genuine use case in some kind of hostile environment. You could probably make one of these very resistant to water/temperature/radiation, with the controller kept safely out of the way.

(And agree with the nostalgia - I'm sure for many the sound is associated with a quiet anticipation or excitement of embarking on a journey.)

In any case they probably won't work upside down.

Add rotary springs (is this a valid term?) to the flap hinges so they're forced down once released from the top edge of the frame.

After a flap has passed the upper lip, which is kind of holding it back to have a precise "flipping point", gravity will take care of moving it down.

No gravity, no flap. ;-)

i.e. places where you want a very large display that's readable in direct sunlight.

If you were equipping a train station or airport in the 1980s, your options were split-flap displays, CRTs with sunshades over them (maximum size ~20 inches), a multi-CRT video wall ($$$$$$$$), a vane display (numbers only) or a flip-dot display. And a lot of places that were equipped with them in the 1980s still had them in the late 1990s.

These days you'd use an LED display, or a sunlight-readable TFT.

Many UK rail stations had them into the early to mid 00s. If I'm not misremembering, the boards at London Kings Cross were not moved to different tech until at least 2005. Manchester Piccadilly had them replaced around 2001 (at the time I was passing through there a fair bit, travelling between York & North Wales).

Part of the problem with the LCD signs is they show too much information. It’s not really an advantage to show the next 30 trains incoming on the big board when only the closest 2 or 3 have gates assigned. They also tend to be themed in colors that are harder to read (Amtrak does blue on white, which is harder to read than white on black). Some stations tried replacing the split flaps with single-character LCD displays to mimic, but they always ended up breaking faster (and never getting fixed) for some reason.

From what I recall in an article about these signs, industrial versions were ungodly expensive to begin with, but the real reason they’re being replaced is that the companies that originally made them are all out of business and you simply can’t get parts for them. It’s cheaper to replace an entire LCD panel than it is to fix one of the mechanical signs once and you don’t have to wait 6 weeks with a broken sign for parts to be manufactured.

And dangerously out of date in airports and train stations.

When they break down they had to be covered to prevent misleading passengers.

- assuming a fixed output selection, a split-flap display can also achieve far greater visual quality than other competitors like flip-dot displays

- defective units can be automatically detected (by comparing the position of the indicator magnet), whereas stuck pixels in flip-dot, matrix or LC displays require a human to spot them

Also, contrary on what I have seen on LED matrices or other RGB/pixel based displays: they won’t display advertisement which otherwise manages to sneak in – too close to information.

You can simply fabricate the flap by cutting the flap to dimensions and affixing letterings onto them. That’s not possible with LCDs or LED arrays.