Biomes describe how a local patch of the world should look and behave. By writing custom biome data you could:
1) Change the general shape of terrain for a biome.
2) Change the ratio of frequency of biome types.
3) Change the blocks that make up the biome, both at the surface and below.
4) Change the distribution of decorative features (e.g. trees, grass, etc.) for a biome.
5) ...and more!