These are just my thoughts, and I can't find anything explicitly and/or officially stated anywhere to back them up, but here's why I believe they chose to use salt for the surface of Crait. IMHO, it mostly has to do with visual effects choices made for the planet and its fauna.
- From a pseudo-scientific visual effects perspective, it couldn't have been snow in order to produce the spectacular red plumes behind the V-4X-D Ski Speeders as they raced across the surface. Snow (being composed of frozen water in crystallized form) would have, over time, gone through a cycle of melting and refreezing, which would have compacted the red mineral beneath it into more of a clay, which would (most likely) not have the appropriate physical properties for making such a large cloud of pulverized "dirt" by simply skiing across it.
- While the white covering could potentially have been sand, a planet comprised of mostly sand (e.g., Tatooine, Geonosis, etc.) probably would not produce a creature with the crystalline look of the vulptex (crystal wolf). Salt is a much more likely "base" for producing creatures with crystalline features. Additionally, sand would generally not produce the same reflective properties as salt, nor would it be as "slippery". Even though it isn't expressly pointed out in the movie, according to Wookieepedia's description of Crait's salt flats:
The white salt-covered surface reflected the planet's sunlight, which would be blinding at midday. The sodium-containing salt also made the planet surface slippery, hard to travel; with gusts of wind sending up flumes of white salt.
Of course, they needed something that would shift under the characters' feet as they moved across it in order to "subtly" give away Skywalker's secret during his battle with Kylo Ren. With snow and sand excluded for the reasons above, salt seems to be the next most logical choice. As I said, these are just my speculations, but they seem to be the most reasonable explanation for why salt was specifically chosen in this case.
As for why they made a point of mentioning it (in answer to the actual question you asked), if the filmmakers were following the same line of reasoning as I've outlined here, this may have been done in an effort to explain the points I made above.
I found the following article from Earther.com that explains some of the "actual" science that could potentially be in play on the surface of a planet like Crait. Titled "The Amazing Earth Science Behind The Last Jedi's New Mineral World", the article talks about the possibilities that could lead to the red color just underneath that layer of white salt:
Malaska [Mike Malaska, a planetary scientist at NASA’s Jet Propulsion laboratory] could think of a few potential explanations.
For one, it’s possible that we’re looking at a very thin coating of salt, atop some sort of bright red soil. “When salts and dirts are mixed and wetted, the salts can dissolve, then get wicked up through the dirt and evaporate, making a nice white crystal evaporite on top,” Malaska explained.
The salt’s crimson color could also be the result of radiation damage—halite (table salt) can turn dark red-brown, and sylvite (potassium chloride) can turn purple when radiation bombardment causes defects in the minerals’ crystal structures. In this case, recrystallization of a thin, upper salt layer has reset the crystal lattice, but the radiation damage—and bright colors that accompany it—are visible right beneath the surface.
Finally, Crait’s crimson salt streaks could be the result of some weird alien microbes.
Plenty of salt-tolerant microbes live in salars on Earth, taking advantage of the ample sunlight and abundant chemical energy available in the minerals. But they have to be UV radiation-tolerant, a feat which certain bugs, like the pink cyanobacteria in Death Valley’s salt flats, accomplish with brightly-colored pigments. Perhaps Crait’s microbes have evolved a red pigment to protect them from the harsh, alien sun.
