A growing number of people in the industry have become aware that chlorides in concrete, in combination with water and atmospheric oxygen, can accelerate the rate of corrosion of steel such as reinforcing bars, steel decks and steel frames that are in contact with the concrete. Many engineers now write specifications that completely ban chloride based admixtures from their projects. Specifying nonchloride accelerators doesn't necessarily guarantee that accelerators will be noncorrosive. Some nonchloride accelerators have been found to promote corrosion on occasion; so if you're worried about corrosion, it's better to require that the accelerator be noncorrosive, not simply nonchloride. Also, admixtures are not the only potential sources of chlorides: mixing water or aggregates can also contain chlorides.
The primary use of nonchloride accelerators today is in ready mixed concrete. Here, in addition to the noncorrosive aspects, these accelerators offer the usual advantages of acceleration: earlier floor finishing, less trouble in scheduling help, and earlier form removal. Precast concrete manufacturers also like nonchloride admixtures because they believe these products minimize problems with efflorescence and discoloration while avoiding corrosion of mesh that has little cover.