To create a crackle glaze, the glaze surface actually develops cracks (which do not affect the integrity of the tile).  As a result, crackle glazes are more prone to water staining in wet areas and must be sealed prior to installation.  Resealing the area should be repeated every 6 months to help prevent or delay the effects of water staining, though water staining does not affect the integrity of the tile.
Sealing is also recommended for backsplashes to help prevent staining in the cracks from food splashes while cooking.  Unsealed tile may stain.  Resealing the area should be repeated every 6 months to maintain best appearance.
See the Stellar installation guide for complete information.