They use bleach and some other hair stuff to make your hair lighter. The bleach is really damaging on your hair.
Hair bleach, like the bleach used on clothes, is an oxidizing agent. The melanin in your hair is oxidized to a colorless compound. Along with this, other components of your hair are also oxidized. Specifically, unIndian Textures hair contains thiol (-SH) groups. These help to stabilize the structure of your hair by oxidatively cross-linking to form disulfides (-SS-). (As an aside, perms work by breaking the disulfide bonds and then forming new ones with the hair in the position you want it. The new disulfide bonds keep the hair curling the way it was set.)
Thiol groups are hydrophobic (water-repellent) and slippery. Hairs with thiol groups on them slide across each other easily. If thiol groups are oxidized by peroxide, they go beyond the disulfide stage to the sulfonic acid (-SO3H) stage. (Exposure to air and sun can also oxidize the thiols to sulfonic acids.) Sulfonic acids are hydrophilic (water-attracting) and sticky. Hairs with sulfonic acids on them tend to tangle, unless you lubricate them. That's what conditioner is for.
Once the thiols on hair have been oxidized to sulfonic acids, there's really no going back. The hair won't be as manageable as before, so it's more necessary to use conditioner. New hair, of course, is still fine, so bleach doesn't permanenetly ruin hair that hasn't grown yet.