Because no hash is eternal and it is only a matter of time between it being leaked and it being cracked. How long depends on a lot of stuff (technology, implementation, password quality and, overall, the value of the account).

Salting, specifically only has one function: making rainbow tables useless and difficulting hash analisys, it is the deffinition of buying you time and making the attacker think twice by requiring more resources (ideally enough that it;s not worth trying)

So, if you know that your users creds are compromised, the only logical answer is to reset them. What you did when hashing is buying time. The difference is that if you bought enough there is little change of incidents from the leak. If you didn't it may get messy. And will.