The diluted bleach water is intended to kill any microorganisms, bacteria, and/or fungal spores that may be on the
seeds themselves that could cause disease to the plant once it sprouts. I like to use H2O2 (hydrogen peroxide) to do this myself. It's easier to rinse off from the seeds before you sprout them. You don't want the seeds to sprout in bleach water or H2O2 or it will kill the roots.