|A different method of painting...|
A quick Google will return tons of information and videos showing how Gansai Tambi colors differ from standard western watercolors, and how best to use them. They have a more vivid tone in many cases, and higher opacity, but they aren't matte like Gouache.
My favorite thing besides the vibrancy and high contrast (you can get the colors very intense very easily, and you need the tiniest amount if you want a light wash of color), is the shade range. They have all the interesting in-between greens and reds and blues that you'd usually need to blend several colors to get in standard paint sets. Ivy green, an incredible malachite aqua, prussian blue, bean red, etc.
It's just EASIER for beginners to find distinctive tones for nature-painting and fantasy art, without fussing with a ton of blending.
Have you played with any of these and which are your favorite palettes or colors?