Rift Lake cichlid 'salt' will increase the hardness and alkalinity of water, which is the ideal (but not necessarily required) water properties to keep them in. However I don't believe rift lake salt will perform as a general prophylactic pr disease preventative.
If kept well (fed a quality diet and kept in a stable environment will great water conditions) they'll be healthy on their own and typically resistant to maladies (barring, of course, the introduction of disease(s) brought on by a new fish that wasn't quarantined, etc).