But let's demonstrate anyway. As said, what follows is probably obvious, but it is also interesting.
The first table gives a bog-standard result. A random encounter with one of 6 possible monsters. The second table is more interesting, because it gives a combination of results through the interaction between columns: 36 possibilities. It's richer. And the third table is richer still: 216 possibilities.
Now, at the most superficial level that provides more variety, which is probably a good thing, all else being equal. Variety is nice. It avoids repetition. But let's drill down a little.
- The second table communicates information vastly more efficiently than the first, and the third vastly more efficiently than the second. There is simply more stuff concentrated in the table as columns increase. Writing out all 216 possibilities for the third table (orcs near a crevasse in a thick blizzard, orcs near a crevasse in a hail storm, orcs near a crevasse in high wing, orcs near a crevasse in a fog...) would take a long time but also a lot of space. A random table with a number of columns is like a mechanism for compressing data ready to be unpacked through the use of the dice - there is no better tool available to the writer of an RPG product for doing this.
- The second and third tables require much less thought on the part of the DM in order to make them interesting. "Orc" requires spur-of the moment creativity. "Orc, abandoned yak herder village" is easier to deal with. "Orc, abandoned yak herder village, in high wind" practically runs itself. The third table lifts the burden of having to come up with interesting things on the spur of the moment.
- Paradoxically, while requiring less thought on the part of the DM than the first table, the third also cannot help but make him more creative. "Snow nymph, abandoned yak herder village, third party involvement" can't help but make him come up with a creative solution - why is the snow nymph there and what's the third party? "Frost giant, frozen lake, fog". What's going on there? A frost giant engaging in impromptu ice fishing, invisible to the players because of the mist - with the added danger of possibly falling through the ice. So the table both requires less creative thought, and focuses it.
Random tables, then, are little packages of concentrated usefulness. They hold compressed information and creative power and release it with great efficiency when called upon to do so by the rolling of dice.