What is this small optical retroreflector used for? Laser ranging from a Mars orbiter should be difficult with the small number of corner cube reflectors.
About the size; we are used to seeing the large retroreflectors on the Moon, but they are at almost 400,000 km away. For orbital distances 1000x times closer, a given beam waist diameter will expand by 1000x less. Of course, from orbit you would not have a huge telescope like they use for the Moon, but it still works out that you don't need as large of a retroreflector array as this one:
above: from here
This isn't the first retroreflector on Mars, though it might be the first useable one.
The Schiaparelli spacecraft unfortunately crashed on Mars with its retroreflector back in October 2017. In the question Could Schiaparelli's retroreflector array (still) be used? I quote Spaceflight 101:
The INRRI Corner-Cube Retroreflector weighs under 25 grams, is 5.5 by 2.0 centimeters in size and is envisioned to be tracked by Mars orbiters capable of laser ranging or laser altimetry, or even laser communications. Its presence on Mars may also benefit future Martian geodesy or general relativity studies. INRRI has an aluminum body with eight fused silica Corner-Cube Reflectors within it, attached by a silicone rubber.
Installing a retroreflector on a rover or lander will enable precise georeferencing during the surface exploration activity. Having precise georeferenced data is useful in tracking sites that may be of interest for future exploration or sample return. Establishing a catalog of georeferenced sites of particular interest is a priority for NASA and ESA in the agencies’ long-term goals for Mars exploration.
The INRRI reflector is wavelength-independent, providing an initial platform to explore ranging on Mars, and prove the principles of laser communications between a surface vehicle and an orbiter.
As far as InSight's retroreflector, they look a lot a like , and according to the text in the linked image page
Below the target in the photo is an Italian experiment called the Laser Retroreflector for InSight (LaRRI). LaRRI is the small, copper-colored dome covered with circles just below the calibration target; it won't actually play a role in InSight's mission. The national space agency of Italy (ASI, for Agenzia Spaziale Italiana) provided LaRRI to be used by a possible future Mars orbiter mission with a laser altimeter making extremely precise measurements of the lander's location for fundamental physics studies and precision cartography.
Both are for future orbital laser missions and both have a link to Europe, and they look identical as far as I can tell, so I suspect they are sisters.
As far as an answer to your question, flights to Mars don't happen every day. Once it's there, it's potentially useful for a very very long time, both from space, and from land and aircraft at Mars as well, perhaps if only for testing purposes.
The Apollo retroreflector arrays turned out to be extremely useful for a half-century, and even LightSail 2 has one for testing purposes. See Planetary Society's LightSail Spacecraft's corner cube reflectors; how large, and corrected for aberration? and also How will The Planetary Society's LightSail 2 Spacecraft's retroreflectors be used?for more on that.
From this answer to How will the corner cube retroreflector array be used on Sentinel 3B?