Board Thread:Theories and Speculation/@comment-25132842-20180320052324/@comment-734582-20180320072822

Erich Zann III wrote: However, does the activation of the Rinnegan prevent further degradation of the user's eyesight from use of the Mangekyo Sharingan, or does it simply reset the user's eyesight at their original potential? Kakashi's sharingan was nearly blind, but once Obito got it he suffered no issues. Obito had six paths power. Indra too seemed to suffer no blindless, and it's implied he had six paths power too ("Same strong chakra as his father"). Since the Rinnegan is also a mean of acquiring Six paths power, it's likely that it ensures the eye will never go blind.

So, it restores eyesight and prevents further degradation.