Because the empower the use with multiple abilities. Firstly, the cells have incredible life force, so even if the transplant subject in question is old, like Danzo, it can greatly increase your stamina and vitality. Also, the cells even have the ability to grant the user Hashirama's Regeneration Ability, which means the user can heal themselves without even using hand signs or medical ninjutsu. What's more, if their lucky, some even develop Hashirama's rare Wood Release, which is an extremely powerful kekkei genkai, and can give the user control over Tailed Beasts. Lastly, if the user is an Uchiha, then they can get access to the Yin-Yang Release, and if they are also host to Indra's chakra, they can will get the Rinnegan.