It is common practice not to store user’s password in clear text. However, if an attacker has seized control of the password database, he is likely already capable of modifying any user data on the site as an administrator. Why bother hashing the passwords then?

To prevent leakage of user's passwords.

Explanation:

I don't really know the premise of your question, but I assume it is this. If the hacker has access to the user's data, they may have the email address. If they cannot crack the password hash, then they may not attempt to login to the user's accounts on other websites.