They have an LED each in the top-right corner of the corresponding dot. The LEDs use different driving signals (much higher frequency and not just when the display changes) but are kept in sync with the slow-updating display to allow both technologies to complement each other: they do work in total darkness and faulty dots have LEDs as a fallback; the LEDs are half-brightness at night, full brightness at dusk and off in daylight.

Also, they were significantly LESS expensive than a sufficiently luminous LED display in the 90s before superbright LEDs existed.

As I said in another comment, they weren’t designed for ads but info signage, so they don’t actively catch attention, which is what you want to get a visually cleaner environment.