This practice is widely considered useful, as rebooting the game will modify hidden data values. The values that determine the shininess of an in-game Pokémon is of particular interest here, as are the Pokémon's IVs and natures. Soft-resetting eliminates the need to constantly turn off (and on) the game when attempting to get a Pokémon with these specific qualities. Although time-consuming, it is a widely practiced method to get a desired Pokémon.
In some cases however, the values are preset, like how it is impossible to get a Shiny Victini legitimately, and N's Gift Zorua from Rood has it's nature set to Hasty.
Source: Experience and a clarification from: http://bulbapedia.bulbagarden.net/wiki/Soft_resetting