Not just corrosion, but also to prevent precipitation in evaporative cooling systems (the most common ones).
Evaporative systems require constant input of new water; if you're adding saltwater the salt will concentrate and it'll become a saturated brine, and once the brine evaporates a bit the salt precipitates. It'll happen mostly on the cooling fills (that will need to be replaced more constantly), but the main issue is that some precipitate does get carried by the brine and clogs the pipes.