Normal video cards work in 8-bit mode. With 8-bits per channel each R/G/B channel can increase in 2^8 or 256 steps from black to R/G/B. In most cases (movies/games/desktop apps) this is more than enough. For some graphical applications like Photoshop you might want to have more "color resolution" so that colors are reproduced more accurately. In 10-bit mode the R/G/B channels can increase in 2^10 or 1024 steps from black to R/G/B.