Saccades definition
Saccades are a rapid eye movement (not to be confused with deep-sleep) where the eyes will make quick, small jerky (hence the name, derived from French) movements, darting back and forth, generally horizontally, even when you try to look at a fixed spot.
Proposed explanations
The Wikipedia page for saccades gives some explanations for their function, including building a “three-dimensional map” and gathering more information by expanding the detailed-vision area since the fovea is relatively small. However, both of these explanations are specious.
Counters to explanations
If the purpose is to build a mental-map, then why do they still occur when you are specifically attempting to stare at a fixed point? What purpose does it serve to have the brain induce an involuntary behavior that opposes your intentions? That’s not very good evolution.
If the purpose is to counter the fact that the high-detail fovea only covers a relatively small portion of the visual-field, then why did we evolve to do this complex and counter-productive behavior instead of just evolving a larger fovea? That’s not very good evolution.
OLED functionality
There’s a more logical answer that makes sense. The retina is like an OLED screen.
An OLED screen works differently from an LCD/LED screen. An LCD screen works by controlling the orientations of many tiny crystals in the liquid layer inside the screen, which can either allow or block light that is provided by an LED (formerly fluorescent) backlight. An OLED screen on the other hand, has many tiny LEDs that are individually controlled to actually emit light themselves, thus providing a much better image, both in terms of color, but also with true blacks since there’s no light being emitted at all, which also has the benefit of reducing power consumption.
Burn-in
Sounds great, huh? So why haven’t OLEDs replaced ALL TVs and monitors? Like with everything else, OLEDs aren’t perfect and do have a downside. In this case, it’s that OLEDs are susceptible to screen burn-in just like good old-fashioned CRT screens. What happens is that if an OLED is left on for too long, it can “burn out” and get “stuck” and continue to show an image indefinitely (like when parents would tell children to not make faces because their faces could get stuck like that, but for real). There are plenty of photos of phones, TVs, and computer-monitors showing OLED burn-in of things like the clock, the news/sports chyron, the Windows taskbar, and so on.
Burn-in remediation
In the days of CRTs, the solution to prevent burn-in (other than turning the screen off) was to use screen-savers which would blank the screen (or later on, display pleasant imagery that constantly changes to prevent any of the phosphors from burning by being active for too long, or at least the proper screen-savers would, there were many that didn’t understand the purpose and showed static graphics that caused burn-in).
In the days of OLEDs, screen-manufacturers try to prevent burn-in at the hardware level by inducing a slight judder at the sub-pixel level. The image isn’t actually static, the screen will jiggle it very slightly, which is usually imperceptible without a magnifying-glass or microscope, and gets less perceptible with higher-density screens since the pixels are even smaller, let alone the sub-pixels (the individual R, G, and B components that make up a single pixel). This way, the sub-pixels are getting a varying signal instead of a constant one, and are less likely to burn in. Of course, this has varying efficacy, jiggling a static white image won’t help.
Retina functionality
The retina works in a similar way. The photoreceptors (rods and cones) will desensitize and reduce their firing rate when exposed to a constant and unchanging stimulus. That’s why if the visual-field is relatively static (like while driving on the highway), one can get “tunnel vision” where the world just turns to gray and seems to fade away, especially in the periphery.
Analogy and new explanation
This is why (micro)saccades exists, to “jiggle” the visual-field a little bit to prevent “burn in” on the retina to keep the photoreceptors firing and prevent tunnel-vision. You can still overcome it by staring intently at a very small point (if it’s too big, it’ll be harder to avoid saccades), and induce the graying, but in normal life, the saccades are what allow us to continue seeing at full-strength.
Anti-debunking
Of course, one might wonder why saccades evolved instead of just preventing the photoreceptors from desensitizing, and that’s because photoreceptors are like smoke-detectors (or Homer Simpson’s everything’s-okay-alarm) in that they work by constantly having a signal firing and input stimulus actually reduces the signal instead of boosting it. This has various effects from simply reducing energy in the default state of non-stimulation to providing for a visual system that works for both detail/color and dark/movement instead of improving one at the expense of the other.
Smugness
So saccades are just nature’s screen-saver. As usual, nature beat humans to it by a billion years.